I think you're being too pessimistic.
For starters, Lucas Moura is effectively "new blood" since he only joined in January and so didn't have a pre-season under Poch. And I think we'll sign Grealish shortly, along with one or two other new players.
I also think that Aurier will likely improve, having had one season in the Prem now under his belt and a full pre-season this time around (he joined very late in the window last summer, on August 31st, as did Juan Foyth).
We'll also have Lamela for hopefully the whole season (he didn't play until late November last time, due to injury). Ditto Harry Winks, who also missed many games last season due to injury.
Then there's the possibility that some of our younger players - Carter-Vickers, Josh Onomah, Walker-Peters and so on - will shine and play a greater part this time around.
And finally, we'll be in our own stadium for home matches, rather than Wembley.
Balanced against all this, of course, is the possibility that we'll lose Alderweireld and maybe Dembele. But all in all I'm OK with how things are currently.
Many fans are obsessed with the transfer window and treat it at some kind of race to see who can sign the most new players and spend the most money. But as Spurs have shown before, there's a lot to be said for continuity and relative squad stability.
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.