It will be quite a while yet before 5.7 will be released. Also, even after it is released, it is always good to wait a while as bugs may need to be fixed.

It is also planned (I believe) that 5.6 will still be supported for a year or so after 5.7 comes out.

So, I would say go ahead and continue with your addon. Never hurts to have more options!

I think in this particular instance the question is very much about economics.

The addon is developed and works quite well. As with most new developments it could benefit from some refinement. However, it also needs to purchase an oem licence for included 3rd party software and that has to be weighed up against potential sales.

Well, as I understand it, it will probably be very low sales as soon as 5.7 is out, and I am afraid this feature had been quite advertised by the core team in pretty much every possible way… so it is something people tend to wait for the upcoming release. (So there is a chance that most people will just wait for 5.7 to be out).

That said, what I would say that some users might want not to upgrade to 5.7 before a while, but would maybe like to benefits from the ease of use of this addon. Moreover, 5.7 do not have any fixed date for release so far, and we have been waiting for it for quite a long time already, so some might just get too impatient to wait any longer to get this type of editing experience.

I would not personally expect to have that many sales, so if you need more than say 20 sales to cover for the OEM expenses, that might not worth it… but who knows.

Anyway, that's only my two cents here, and it does not worth more than my personal opinion.

Hope it helps.