It goes both ways; there may be a lot of sub-par lazy professors out there, but the fruits are so valuable that you can see it as buying a lottery ticket that actually has a positive expected payout. It's hard to picture a world where breakthroughs on difficult problems and honest research that lead to controversial findings don't exist, so we get too used to it - and forget these can't be allowed to happen if they are hindered by poor academic culture and policies.
A lot of professors that work hard are protected that way. Old professors also do not have to directly compete with young researchers, so it's much more likely a committee would make sensible faculty hiring / tenure decisions that are free from conflict of interest. It's another issue that academics have to deal with other kinds of competition, such as research funding, as well as erosion of the tenure system by e.g. abusing adjunct professors.
There do exist professors who switched to less hype-driven research topics once they got tenure; for the most convicted, it's part of playing a game to get their messages through. As you can see it's already a hard game to play, so we don't really want to make it even harder. It's also easier (and quite common) for researchers to move on to more difficult topics after they get tenure, where doing it before tenure is close to self-destruction. Andrew Wiles used his tenure status to gain 7 years of solitude, slow publishing small pieces of research he accumulated previously, just enough to avoid getting fired, and proved Fermat's last theorem, a centuries-old open problem. And there are actually more of these people than you think, that are willing to play some games while working hard even when nobody's watching.
It's also not so clean cut that you can only do popular research to get tenure. Having a "good old boys club" can do bad and do good; a bad academic culture is harder to attack, while good academic culture is also more easily preserved. At a place that has a well-established healthy culture, it's easier to do `unpopular' research and still get tenured, and keep it that way. Sure these tend to happen in top institutions with an abundance of resources and strong attraction to top researchers, but it can be sustained partly because there are strong protection mechanisms within these institutions.
Does the tenure concept need to be refined? Probably. Does it bring good to all universities? Probably not. But it is one of the strongest foundations of a thriving academia, which far-reaching effects, like a bottom piece in Jenga, so you need a view of the big picture and bring a much more stronger argument before you can take it away.
