The Match monopoly buying you out is one problem. A bigger problem is that dating sites have to have a lot of users before anybody wants to use them. (They especially need women users.) This conundrum is a huge barrier to entry in the space. The temptation to pad your user base with fake accounts, scammers, and sex workers is tremendous. If you do that, you boost engagement for a few months and then you die.

Yet another problem is churn. If people find a life partner they drop off the site, and you constantly have to find new users to replace them. This is one way a dating site is fundamentally different from e.g. Facebook. As a result, some of the less-scrupulous sites purposely make it less likely that you will see people who seem like great matches, or they interfere with your ability to message those people, just to keep you using the site longer.

Jump-starting the initial chicken/egg issue in an ethical way is AFAIK an unsolved problem. In the post-OKC era, Bumble is the only one I know of that semi-succeeded because they made it woman-centric from the start.

Finally (yeah, I've thought about this a lot), the modern mobile app-only approach can be a distinct turnoff for people like me. Mobile apps make it difficult to write much text about yourself, which was something OKC encouraged. Apps also present a safety issue: Apps often require location permissions, which means employees of the site can stalk women. (This has happened.) There's no good reason dating apps should require location permission, but most do for the same reason most apps of any kind do: Location gives the site owner personal info they can sell to advertisers. All that having been said, the vast majority of the market probably prefers mobile apps to web sites.

The whole space seems like a cesspool of bootstrapping problems coupled with extreme incentives for unethical behavior. Sorry if I sound pessimistic but you asked what's stopping this from happening. This is my take.