As one of the primary developers of the Subscriptions plugin, this was not designed to be extended with other payment services. If support were to be added, it would need to be a thoroughly vetted and well-thought-out PR to the official plugin or a fork would need to be maintained. @pfaffman and @spirobel are right in that it will take a non-trivial amount of effort to get this done.
That being said, Stripe is investing in crypto again.
There may be BTC support on Stripe in the near future. Adding that to the plugin would likely be much simpler.