I do these routings all the time for business trips to Europe as my corporate travel policy doesn't not allow paid J, but I can buy upgradable Y if it is within +/- $2K of the lowest Y fare for the routing. It all depends on what you want - I like sleeping and not arriving like a total zombie, hence the lie-flat from YYC/YUL/YYZ to Europe helps me arrive more refreshed and less likely to fall asleep before dinner at my arrival destination.

If you don't want to risk it with an M fare, why don't you just pay a little bit more for a B Lat fare and guarantee the upgrade? That seems to make way more sense to me.........$1300 K each way from DUB-SFO and return, can confirm you in J for <$3K when then J fares one-way are $5200K+......

Both LHR-YYC/YYZ-SFO and the return on AC metal are showing plenty of R space right now, so you'll have a guaranteed upgrade rather than playing Aerolotto with an M fare.

And honestly, you really shouldn't be criticizing a system that allows you an opportunity to take advantage of a $10K return ticket for <$2500K return on an M fare, just because you have to wait for 4 days to confirm. They've given you several other options to obtain that J seat, and in these case the Lat fare is a no-brainer if you really want it. Not only is it a cheap way to score a lie-flat J seat, but its also refundable in case of last minute travel emergencies. Makes complete sense to go with a B fare vs. M fare.

You want a guarantee, by Lat fares on flights with R space or by a J, C or Z fare.

Want to save cash? Sit in Y or play Aerolotto. Or use a SSWU on your cheap fare.