That seems wise. But during construction, Union-bound riders using the 509 and 510 will then have to get off at the nearest stop to Union (near Simcoe). This would leave something like a 750m walk to the subway. I'm not averse to this as a possible temporary solution, but I think that kind of distance may cause it to be ruled out. As well, the only loop on QQE that I'm aware of will be at Parliament, which I guess is fine. And it'd be interesting to create a new cross-waterfront streetcar route (albeit a temporary one), with the 509/510 going all the way to Parliament. But again, with Union being over 0.5km from stops on QQ east or west, this may be seen as a less favourable option.
If service disruption at Union can be avoided, I think it'd be worthwhile. But no doubt cost will be an issue. As it stands, cost is already the greatest issue - so any which way I look at things, the QQE plan has problems. My stance is that factoring in these hidden/newfound costs to the original project cost, perhaps we could use the same amount for a different solution (one that was originally ruled out, or a new plan altogether). And hopefully a plan than won't involve future riders/tourists being corralled like cattle and having to wait in long queues at Union just to access the waterfront.