To be honest, I think there is a certain naivete in expecting rules to make senses in all cases/never to be demonstrably "absurd" in precisely the cases where it will marginally fail.

I mean what about the idea of closing the gate 20 minutes before boarding? (Some airlines even claim earlier times). Imagine - you arrive at the gate 19 minutes before, gate is closed but plane is still there, you have no big bag, yes of course, you could be onboard in less than 2 minutes, plane would not be delayed, in fact, maybe you will then find out that even though you were refused boarding the plane was stuck at the gate for 1 hour because of an ATC delay or whatever (has nobody seen those videos of two guys forcing their way through the gate a few days ago and trying to jump off the jetbridge onto the tarmac as they thought it was ridiculous they had been denied boarding??) Why have an MCT increasing to 90 minutes when for many of us experienced GCH and GGLers we'll mostly make it before boarding ends with 45? Why is it ok that a valid 85 year old on their first flight who speaks correct but not great English is allowed to sit in the emergency exit whilst a 15 year old who has been commuting on a monthly basis for years and actually listens to instructions is not? And I'll skip the permanent debates on bags through checking, being denied bag checking for your flight at many outstations when the one before yours is being checked, not to mention all the rules on standby on an earlier flight and what not.

All rules make sense until they don't. They are - by nature - crude instruments which people in charge of flows and operations to try and fix a problem (in the case of conformance, delays) by trying to decide where to create a check which will minimise later problems.

So why was conformance decided? Well I wasn't privy to those discussions but let's think of a few points here:

- First, any policy will likely be on some sort of round number so 20 minutes, 30 minutes, 35 minutes, etc
- Second, T5 is big and complex. You have A, B and C gates the latter taking a significant amount of time to get to especially if you don't know your way
- From that point of view, I would argue that the vast majority of passengers on many days who would scan their BP 30 minutes before flight time would not be at their B or C gate when boarding closes 20 minutes before departure time. Heck, remember that at the time you scan the BP, you haven't even gone through the security process, and that alone will often take more than 10 minutes, so chances are that even many current t-35 pax miss the 20 minutes theoretical gate closure
- I know the answer will be "ok but why conformance at all, why not let them take a chance and if they arrive at the gate too late, too bad for them" - well, there are a number of reason why an airline may prefer to know at security point.
1) One is that if someone has missed the flight, it is much better to have them stuck landside than airside - if they are airside, chances are you'll need to bother staff to escort them out of the terminal again, this consumes time and resources in unnecessary ways.
2) It is better to know per se. Hundreds of people do not take their flight every day. With conformance, Gate agents have an up to date list of who to expect on the flight, without it, they don't. How do you know if the person on the list is about to arrive or has never intended to show up? I know people say things like: "oh but that's fine, just close the gate promptly and that's ok" but frankly, that's quite unrealistic. The exact same argument people want to make for showing a little more flexibility at conformance could be made in exactly similar ways for gate closure. You have a Prem flying F and you think that they will feel grateful because you refuse them boarding at t-19 whilst they would have felt outraged had you told them they can't go through security at t-34? That's not serious. And in exactly the same way staff may well show discretion in that case when your Prem in F is arriving at the F wing at t-32, they would be tempted to keep the gate open a bit longer at t-17 just in case... With conformance, if your Prem has not cleared security at, say, t-30, they know they won't need to wait for them and the gate operation - probably the most time-sensitive of all - can be closed promptly if necessary and possible;
- That by the way is important: Landside, resources are "pulled" across flights: same check in counters for many flights, same security checkpoints for all flights, etc. Gate is different it is personnel dedicated to one specific flight. Keep them 1 or 2 minutes longer there and it is 1 or 2 minutes later that they will arrive at the next gate where they are due to manage the boarding process. How many of us have been infuriated in recent months when flights were delayed out of LHR because of insufficient personnel at the gate?
- for 1 person who misses conformance due to a terrible and unpredictable event beyond their control, you probably have 20 who miss conformance because frankly, they cut it too short, should have left earlier, "thought" they were going to be fine because they sort of assume no major traffic, no train delays, and mostly green lights. ultimately, people cutting it short have their appetite for risk impacting the experience of the far, far more numerous people who want to be extra careful when going on a flight and plan to arrive at the airport 2 or 3 hours before the flight just in case. That's not fair. The 35 minutes conformance at least gives a clear indication and I suspect that without it, many people who cut things short would cut it even shorter thinking that if they arrive at t-30 they will really be fine. Those, incidentally, are the people you see pushing in front of you at security because their flight is about to close.

There will be failures with the conformance system as there will be failures with any system. There will be cases of demonstrable absurdity as there would be with any system. But for all practical purposes, if you stop for a second and consider it, expecting people to clear security from landside at t-35 is certainly not an absurd deadline even for the most frequent travellers among us eligible to the fastest security and able to get quickly to where they need to, let alone for random pax using normal security or the often busy regular fast track and who have no clue how to get to C gates. You could then say "but why not have different conformance times for F/OWE pax // for pax leaving from A gates // for whatever" but that is simply not how regulation works, and again, even though i tend to be early at the airport whenever I can, I have seen many people facing that over the years and in my experience, if the case is reasonable, staff will usually call the right person and try to negotiate access as a matter of discretion.