Well, if they pay for the support and their request falls into an agreed-upon SLA, why not? This allows improving the software, keeping it open source and may provide a living wage for the maintainers all at the same time.

Yeah I agree. But maintainers don't always have time to support software, and can't really leave their jobs until they earn enough.

Maybe they could outsource support, any idea if something exists for this?

It's up to the maintainer to offer up an SLA and price it accordingly. Try to figure out the SLA you get for your windows or office license: Many companies would already be happy with "our payment places this otherwise uninteresting issue that we desperately need fixed at the top of the queue", even if no guarantee for a time frame was attached. If a company expects 24x7 with short response times, price it accordingly. If you find customers willing to pay for this, great, you just validated a market :)