There's no equivalent to the "child labor" in your analogy - and I don't mean that because upcharging for video games isn't morally equivalent to abusing children, I mean that because in this instance the potentially "exploited party" is the developers you are proposing a boycott of. There isn't some third party that Unity and the game devs are colluding (directly or indirectly) to exploit, where a "boycott by proxy" might have made sense.

If the dev studios feel like they're being mistreated by Unity, they are already incentivized to abandon it. If they don't abandon it, they either _don't_ actually feel mistreated, or they are just not in a financial position to switch over just now. Boycotting them in either scenario is not helping anyone.