Punctuality is good. So we can blame the design of T5 by not having holding pens at the gate and therefore requiring T-35 at security

It's unfortunate that pushback seems to be the measure of punctuality. It would be more useful if punctuality was measured by "wheels-up" times, thus allowing management to focus on all the other causes of missed punctuality ("where's the tug driver", "APU's not working, waiting for some jump leads", "waiting 30 minutes for a slot even though we pushed back at T-5", or "waiting to cross active runway" etc.) which are conveniently ignored.

If the objective is to claim X % punctual "departures", then sure, pushback is a good measure. If you're trying to get your passengers to their destinations on time, then wheels-up would be a much better one.