Lucas Moura looked decent, but what are we expecting here? He's a Brazilian Aaron Lennon. Grealish is nice but he will only add depth and potential and is far from a sure thing, you can't rely on these sort of players to come in and make a difference consistently. Sure, you might strike gold with an Alli but it won't work every time. Who are the one or two other players? Unless they're players ready to come in and make a real difference it's not enough. We were promised proper reinforcements, Poch was promised proper money to spend. Where is it? Going on Grealish?
Aurier isn't very good. At best he'll be a decent backup for Trippier, who will clearly be our first choice right back. Again, him slightly improving isn't going to have a big impact on our season. He was disastrous at times, got bored of watching him take a terrible touch then lunge in to make up for it. Massive improvement is needed if he's to be an asset rather than a liability.
You're basically counting on everyone coming back and immediately being useful. Winks will need time and is still in need of a lot of time to develop, he's not ready to come in and start. Lamela is Lamela, he'll run around a lot but mostly be frustrating. Decent squad players but again, they aren't going to win us anything, sorry but Winks coming back is not going to be like a new signing. We'll also have new injuries coming in and having an impact, you're looking at a perfect start to the season where all our injured players return and no more injuries pop up.
This is just blind hope. You could say the same about every other top club. Of course a youngster could come in and make a difference, probably not though .. same with United, Arsenal and Liverpool, who all bring in the odd player regularly (Rashford, TAA, Iwobi etc) from the youth team, but it doesn't happen often. Again, you can't count on your youth players to make the difference, it would be incredibly risky to rely on them.
What difference will this make, really? Our Wembley record ended up being pretty good. This is just adjustment to another stadium which may take time, it's not immediately going to feel like home. It's not going to be WHL straight away, sure we'll get a boost at first but it's not a magical wonder super stadium which will make our team better than it is.
We lose Alderweireld and Dembele and only make the changes you suggested and we're absolutely fecked. We badly need to replace the latter with a proper central midfielder (spend some money, Levy) and obviously would need to sign a decent replacement for the former. We also need more competition in the forward positions but seem to have wasted our time sniffing after Martial. I'd even suggest we need to sign a decent backup for Lloris, Vorm isn't cutting it anymore and Lloris was incredibly dodgy at times last year, he needs a bit of competition. Clearly too comfortable, at the moment.
Overall, I think you're looking at the situation with rose tinted glasses. Most of the stuff you said would be true for other clubs (players returning, youth potential, new signings bedded in etc) - look at United with Sanchez, Liverpool with Van Djik etc. Except they're also fixing their issues by spending money on genuine quality, whereas we are sitting still and doing absolutely nothing. It's not good enough after what was promised and I guarantee Pochettino is furious with the current situation.