I think there is a some sort of catch-22 going on. Matrix Foundation can't fund the replacement of Synapse with a better-designed server, because hosting matrix.org consumes all the money. And it consumes so much money because it runs Synapse, which uses computing resources in a very inefficient manner.
And the Foundation might have a some sort of conflict-of-interest, since it is closely interwoven with the New Vector company, which business case is being able to make Synapse work despite its flaws.