I unfortunately had to sell most of my retro games when I was going through a financial hardship, so the only older Pokemon games I still have are Black and Black 2, and the digital version of Pokemon Sun on my 3DS. Luckily I had already put most of my older gen Pokemon into Black 2 and was able to transfer my Pokemon into Pokemon Home so I have a mostly full living Dex (missing a few mythicals like Zarude and Hoopa because I wasn't playing when they were distributed).
Personally it's hard for me to go back for older Pokemon games just because of the HM system. Needing a Pokemon for Cut, Flash, Rock Smash, Surf, Waterfall, Fly, etc limits what kind of team you can build, or makes you have to switch Pokemon out constantly which can be inconvenient if you're not near a Pokecenter to access the PC.
In gen 7 you had ride Pokemon which did all of the HMs for you, and the newer games have a similar set up. The Switch games also allow you to access your box from anywhere which is a huge QoL improvement.
Personally it's hard for me to go back for older Pokemon games just because of the HM system. Needing a Pokemon for Cut, Flash, Rock Smash, Surf, Waterfall, Fly, etc limits what kind of team you can build, or makes you have to switch Pokemon out constantly which can be inconvenient if you're not near a Pokecenter to access the PC.
In gen 7 you had ride Pokemon which did all of the HMs for you, and the newer games have a similar set up. The Switch games also allow you to access your box from anywhere which is a huge QoL improvement.