When Lyft launched here in NYC it was pretty dead, and they lost the initiative with the initial batch of riders. They've since rallied around driver recruiting and now the service is good. Everyone I know here has multiple ride hailing apps installed, and multiple services are competitive with Uber re: driver availability.
So yeah, it's not trivially easy to get into this game, but it's also not absurdly difficult - you need someone with deep pockets, but not that deep. This is especially true because Uber's network effect between markets is pretty minimal - people mostly take rides in their home market, so it's easy for upstarts to compete for a single market without the pressure to take themselves global immediately.
One structural weakness on Uber's part (and Lyft's, too) is that their commissions are still quite high (20-25%?) and this gives a lot of room for competition. Juno launched here successfully simply by undercutting the commission.