I'd have said the opposite, but you raise a point especially with geriatrics Isner, Querrey, and Anderson. I'd say serving well really helps them all plus wisdom on how to get the most out of their game.
Returning is another matter, but some decline in that area hasn't hurt these players that much. Overall I'd say superior serve games are the common theme and that is something that is never easy for the youngest players (even 24 years and younger.) When you think about it is rare to have an incredible serve game at a young age. Exceptions:
1. Roddick
2. Sampras
3. Shapovalov
4. Kyrgios
5. Goran?
Its really a short list and I'd say there success is due in part to overall weakness of the LostGen players on serve (age 25-29). Raonic has a big serve, but he's slightly shaky in crunch time. Dimitrov probably owes his recent success to his serve game, but he's had a lot of bad history in the crunch in his career.
Looking through the rest of the top 50 in the ATP Race (Querrey is outside top 40, but the other big names you mention are top 10

), its not a pretty picture for the elders:
13. Djokovic - and we know his situation is very much elbow/serve related
17. Fognini - has been playing quite well for almost a year and perhaps has the ground game to enjoy bashing with the younger players.
23. Gasquet - fighting back issues off and on so that accounts for his drop
26. Seppi - pretty tall guy and serving well
27. Monfils - injured, but may have his best serve game right now. His pushy style perhaps not the best these days.

28. Chardy - not short and serving well
31. Verdasco - very much like Fognini
32. Kohly - in for a good bash
33. Simon - slipping and you'd have to say pushing and weak serving not helping
41. Berdych - becoming like Gasquet with the back issues and not serving better and have been enjoying his choke jobs.

150+ Tsonga - injured and amazingly after something like 5 titles in 2017, its rather amazing that such a big server is on the verge of dropping from the game.
Really not a pretty picture for most of the 30 something players in general bolstering your OP's contention that this is something unusual. Generally I'd say for a tall player if they don't produce solid return numbers by age 24, then they'll never be great because their return will just become weaker and more marginal. If their really is something to this one may have to rethink Zverev's chances of becoming a double digit slam winner.
I think the gig may be up to some extent for your lumbering giants post Wimbledon. Even though they have traditionally excelled on outdoor hard courts. In the last 52 weeks, it has been NextGen that has been making the biggest in roads on outdoor hard courts. It will be interesting to see the clash for sure this Summer.

Speed kills and if these guys can get broken some that may be the difference. The field is getting a lot younger and faster all of a sudden.