But that's just the problem: programmers don't want to pay for such a tool. I personally see it as a part of my work and having to pay for that feels like microtransactions in a game to me.

So not sure how such a thing could be monetised properly. Likely by only offering enterprise-grade services: the tool itself is free, the customer's programmers could employ it to achieve migration but the customer prefers to outsource the cost. Sort of how several Linuxes are free but come with paid support contracts.