Technically, GameTap had some really neat little features. For example, it would track your high score for most emulated games, and for really old games where the score would rollover to zero, it noticed that and would let you see your effective grand total score. So there would be a global Galaga leaderboard that could happily go into the millions.
Regarding the success of the service, Gametap was live for a few years. It totally had its shot. GameTap was regularly advertised on TV. It had a pretty big library covering a dozen or so platforms: Several Ataris, ColecoVision, Intellivision, Sega, PC games, and more. They did a few high profile things like buying some failed MMOs and keeping the servers running for all GameTap subscribers.
At the end, I think it turned out that the folks who get really excited about playing ColecoVision games are the same folks who are very comfortable downloading ROMs.
I think it may just have been ahead of it's time in terms of model in the era of battlepasses, paid online and gamepass, as well as monthly paid streaming services in general.
Here's a fun technical secret about GameTap. Several of the companies that we bought licenses from barely knew they owned the games and definitely didn't have any original binaries or source, and for the obscure consoles/titles, we sometimes could only find cracked versions online. Those would usually have crack intros (it was the birth of the demoscene!), though, and we clearly didn't want to use them. Ultimately we cheated. We just launched the game by loading a save state just past the crack intro.