It's not the loss of features that made ORAS trash. It's that they dumbed down everything and made it toddler-easy.
If you want to blame someone for that blame Game Freak, IIRC Masuda said that part of the reason for dumbing down ORAS (as well as replacing Emerald's Battle Frontier with XY's Battle Maison) was to keep kids' attention and compete with mobile games.
Catching Latios/Latias as part of the story?
This is something I can
kind of understand from a design standpoint since it's meant be a quick way to give players access to soaring without frustrating the player via accidentally fainting Latios or Latias and having to reset (or undergo a scene that restarts the battle).
That said, you still have access to soaring even after trading, transferring, or removing the Pokemon in question from your save file.
A free shiny Rayquaza for just purchasing?
This kind of thing (getting a special version of a highly-competitive Pokemon for free via an event) happens every now and then and I'm not too surprised by it nor am I going to complain about. It's worth noting that this specific Pokemon was given out to promote the Pokemon TCG's Ancient Origins set, where Shiny Mega Rayquaza pops up in the set as an Ultra-Rare card and as one of the Pokemon featured on the set's pack art.

Every poke that's stronger than a roll of wet paper towels gets a Mega Evolution?
Mega Evolution as a mechanic was a mostly-flawed concept from the start that really should've been given several more passes and edits before being approved. I do agree that there are some Pokemon that really didn't need Megas at the time (Like Lucario, Garchomp, pre-nerf Gengar, Blaziken, Salamence, Slowbro), some Pokemon needing them but should've been given a few more revisions (Kangaskhan, Mawile, Sableye) some that actually fixed their issues (Beedrill, Lopunny, Aerodactyl), and some that just kind of seem pointless (Abomasnow, Absol, Audino).
What's the point? Where's the challenge? Even my 12-year-old self would be insulted by how easy it is.
The point is to keep a continuous stream of younger players hooked to the IP while older players either stick around for parts of the IP (VGC, the TCG in general, teaching other players the ins-and-outs of the games) or they move on and live off of nostagia. It's a sad state that I, and hopefully other age-old Pokemon fans, have made peace with.
Could the mainline games give players some sort of post-story challenge? They could, we've seen it happen with PLA's post-game Volo battle and (sadly) BDSP's Gym Leader and E4 rematches. The problem is that Game Freak and TPC
need to be
willing to step out of the cut-and-paste bubble and actually shake things up. And if you ask me, it's almost as if they're
afraid to do so thanks to the thorough bashing Gen V got.