Most of the small to mid size companies that I have worked for would choose a larger more established system that costs more even if it offers less over a self-hosted one that they had to pay some sort of fee for.
Is like this weird idea in the business world that if you’re using Foss systems that it must be completely free, and that the reason why you are using it is because you are broke or cheap.
My guess is that if you’re going to start a MSP you can do that with Foss and probably have a lot of success as long as you’ve got the sales chops to get the contracts.
Then you can funnel some of your customers money to foss well also increasing awareness and adoption of the better free and open source software programs