I see two sides to this:
1- the internal accounting system. This is not really different from the traditional world, it can be done with most accounting services already. The company I work for (OneStream) focuses on consolidation/planning rather than low-level general ledgers, but handling crypto would just require to use a custom currency (I think we allow up to 7 or so). Managing exchange rates can be done in actual code, so you’d just set up some api calls to decide how you want to calculate those rates and be on your way. I honestly don’t see anything crypto-specific here. The only annoyance would be seeing loads of “0.00000...” in a naively-setup system, but that can also be dealt with trivially in most services.
2- external integrations. This is really where the hole in the market is, I guess, and probably what motivated you to explore the space. Services that will automatically pull your transactions from cryptocurrency exchanges and similar hangouts, might be thin on the ground today; I honestly don’t know. But consider that, for the likes of Xero, because of point 1 above this is basically just a bunch of plugins that are fairly trivial to write, should they decide to steer. If they solved “legacy” banks, they can solve online banks pretty quickly.
So, would a product focused on 2 be successful, in a market already crowded that can probably self-extend to crypto fairly easily? I’m skeptical, personally, but I might be wrong. I’m also skeptical that significant numbers of businesses will go “cryptocurrency-first” and look for such a service. Cryptocurrencies are more like commodities than actual currencies, and I bet no company is “gold-first” in 2021.