And, I find it mightily ironic that you're using this argument to defend FRLG when FRLG were made for the sole reason of, wait for it, compatibility.
This is misleading. Compatibility, which could just as well be made up for by including Pokémon in a game, wouldn't necessarily lead to a whole sequence of two games, being by itself a fairly minor point, which were rather also there to bring attention to those games, possibly update them, etc. 'Sole' may overstate the case here. In any case, FR/LG is other than this just an 'updated' version of R/B, while the current version is not, so if people were so drawn to the newer games they might well think that FR/LG are better games, whether or not they also find R/B useful - and presumably that would depend on how much they care about completing their Pokédex in S&M. It's not reducible to compatibility, and may be more entertaining, just as S&M may not be reducible to its abbreviation, if less entertaining. They didn't just try to put in the R/B Pokémon, and then change the graphics, features, etc., by accident.
That's a flawed logic. Why not play what you call "latest and greatest" then? We are talking about the game itself, not what's compatible with. That's an extra.
To be fair, you suspect that's mostly just phrases, Sun and Moon aren't even released yet. If someone liked R/S/E, of course, then they needn't also like Sun and Moon, but may prefer FR/LG, in which case most of this does not apply. FR/LG is, of course, a more modern game than R/B, so if that's what you're looking for then it's hardly out-dated.
Touching on Elysieum's post, the 'latest and greatest' is what you'd be paying for, presumably, so like if you really want to encourage the team of Sun and Moon...
Some people who like R/B might be alright with them taking a break by this point.
It might be possible for people to consider R/B and FR/LG as games independently, without feeling a compulsive need to bypass these entirely to transfer to S&M. If people were just settling for a lesser game, but they could transfer things to use them elsewhere (and not receive more modern types, generally, so this is mostly one-way), then they are settling, and this doesn't worsen the other games. It just becomes a feeder game, which is surely a lesser role for a game to play. It seems a bit strange to think that new features and graphics that are likeable have only been added since D/P.
So...? The games don't need those (specially Mega Evolutions), and nothing has changed in that regard. The situation is the same as last month/year/decade.
By R/S/E, etc,. the games did have new features and graphics, so it seems a bit unnecessary to just point out that they didn't have the features that they don't have. If you were playing for features, then FR/LG have a few new ones. You wonder if pressing both R/S/E and R/B into just being suppliers of Pokémon for the modern versions isn't a bit degrading, though.