The main issue motivating the formation of Bitcoin Unlimited was the small-blockism of Core. The rallying cry was unlimited blocksize. That original issue is finally resolved, because there is now a version of the Bitcoin ledger whose miners seem hellbent on increasing the available blockspace way beyond the amount currently being used, and at least beyond what most BU members think is sufficient (many even think it is too much). I happen to think this is the correct path, but whatever your view it suffices to note that there is no longer that issue of the blocksize being held down.
The other
raison d'etre of Bitcoin Unlimited was to have an implementation to compete with Core. While the first issue is no longer relevant, the second issue of having competition among implementations arguably remains. SV and BCH both lack competition in their node client implementations.
The hazards of a monolithic node client implementation are familiar from the experience with Core:
1) at this still-young stage, miners can easily end up being railroaded into protocol policies they don't agree with, simply out of fear of bugs if they were to suddenly try to figure out the one monolithic node client's codebase so as to safely mod it themselves
2) any bugs affect everyone (in fact two competing clients exacerbate the problem due to incompatibility, but each additional client mitigates it as any bugs in a given client are likely to hit only a minority of miners)
3) optimization is not incentivized
It seems to me that BU could remain relevant for BCH node development on all three points (in theory at least), and for BSV node development on points (2) and (3).
SV miners appear committed to restoring the protocol to its original functionality, sunsetting P2SH and a few other Core "innovations" before locking it down completely. This removes the very position "protocol dev" from the BSV landscape. No innovation in the base protocol. This is intentional. Once the restoration is complete, the clear Schelling point of setting the original protocol functionality in stone removes all the bickering and the ability of devs to railroad miners as Core has so successfully done for the past 5 years. BU actually remains relevant for point (1) in BSV only until the protocol is fully locked down.
For point (2), BU could provide redundant failsafe variation, and regarding (3) it could keep nChain on its toes as far as client scaling, at least until miners mature enough to all have their own client dev teams.
In the original Bitcoin paradigm with the protocol being immutable, which SV miners subscribe to, there are no protocol developers. Instead there are client devs, and devs building on top of the protocol like
_unwriter*, along with wallet devs, etc. Client devs will just optimize performance. Eventually client developers will probably all work for miners, especially as miners are weaned off the block reward and increasingly rely on transaction fees. This maturation process brings the end of volunteer development and maybe even the end of unaffiliated node development groups like BU. Or maybe not. It depends, but one thing is certain: the future of Bitcoin is in professionalization.
Thanks to the impending halvings, miners
will start competing fiercely on their node performance and connectivity in all scenarios where Bitcoin survives the coming decade. Not Core vs. BU fierce, more like Apple vs. Samsung vs. Google vs. Amazon fierce. Lawyers, industry groups, closed source, open source, patents, ISPs, archivers - a whole new industry will sprout up and old ones will be disrupted. Governments will have their say for the time being, as they get hooked on the blessings of a Bitcoin-powered Internet just like they did the original Internet.
Thus the surviving organizations will be those capable of taking all this into account. Even those building on top of the protocol will usually need to have their works informed with some understanding of the law surrounding money and communications, as well as of course economics and business.
*
At the risk of stating the obvious, there's no reason developers who want to help Bitcoin succeed have to be working on node clients. They could be building things on top. It seems a lot more up the alley of devs who want to innovate. The blocksize cap and the power this gave Core created an anomalous situation where protocol development and node development were of supreme importance. We needed devs to step up; their service in creating competition for Core deserves our eternal gratitude. As a locked protocol, however, Bitcoin needs builders. Those who build things using Bitcoin. Could BU and its considerable funds be put toward innovations on top of the protocol, voted on by members?