I'm not generally fond of the execution overall, since too many I personally dislike (or used to dislike) are those that ends up being too gimmicky or weak to me for reliably use.
It makes sense why single-staged Pokémon exists; they serves as "pre-promotion" for early game or mid-game and aren't made to catch up to lategame, encouraging players to swap around accordingly. In practice, many people ends up being too attached with the ones they caught, but there are many that ended up disappointing, which indeed encouraged them to swap out.
A look onto a Smogon Forums' Orange Island thread on in-game popularity will speak volume on how much people are not really willing to give single-staged Pokémon a chance, with only Laprass, Snorlax (in Gen 1 and the remakes), Heracross, and Mimikyu managed to reach the top ten. Many of the single-staged Pokémon ended up among bottom ten (or less depending on the game) for various reasons.
And even if the single-staged Pokémon are barely more appealing to the audience due to being new, same cannot be said with returning Pokémon that remained single-stage, causing the audience to become increasingly unhappy with them. And while each Pokémon has their fan, popularity is an important factor on if the Pokémon is welcomed with open arm or if it is just a hopeless filler no one asked for a return or focus unless they are expending upon it.
Not even later generations are safe from getting detested single-staged Pokémon either, with Bruxish and Stonjourner being the worst of them all.
The existence of too many single-staged Pokémon that are way too gimmicky for their own good, in example Castform, Spinda, pre-Gen 5 Ditto, or flat-out duds, with Volbeat and Illumise being by bottom two amongst them, even than Luvdisc, overshadow the goods among them like Zangoose vs Seviper, a good amount of Gen 7 single-stages like Oricorio, Aerodactyl caused me to have a very negative opinion about single-stage Pokémon, especially weak-looking ones, due to overall awful execution. If you wholeheartedly disagree, more power to you.