At the expense of creating a trust assumption about relays, and there will objectively be far fewer relays than validators. The most likely outcome is one major relay, with a handful of minor and/or special-case relays. Network effects will cause one relay to become dominant, because MEV is all about maximizing rewards and any relay with an advantage over the others will pick up the lion’s share of MEV (and hence the lion’s share of validators chasing said MEV).
That is up to the searcher to decide. They can put their bundle through sooner with higher risk (but less chance of losing their MEV to a competing bundle), or later with a validator they prefer. If a validator has little history, however, then it seems a fair assumption that the validator is unlikely to have built their own internal infrastructure to take the proposed block, decompose it, find the MEV, extract it, build a new block, and validate and broadcast said block on the off-chance that a long tail MEV opportunity comes along just at the right time.
Alternatively, given that a relay will be in play all of the time it would be easy for that relay to be 99% trustworthy and subvert the occasional bundle: it has both the financial incentives and the power structure to be able to do so, and it is easy for parties to believe that a small amount of MEV going missing doesn’t point to a corrupt system.
A general point I would like to make, though: the trusted relays design helps to maximize MEV returns at the expense of decentralization (in operational terms), stability and diversity of the network. Trusted relay creates a chokepoint through which all (or many) block proposals will flow, both for selection of the payload and broadcast of the block. Consider: in the trusted relay model a corrupt relay can stop block creation (for one slot at least, and highly likely for many more) acting against the validator’s interests, whereas in the trusted validator model the worst a corrupt validator can do is steal MEV. As far as the chain is concerned, the former is far more problematic than the latter. The chain is also able to punish the validator (both within the protocol and socially), but has no similar capability over the relay. Fundamentally, relays are external entities and so should not have any significant level of control over what happens in the network.
Decentralization is not simple when there are conflicting views of which bit of it matters, and the course seems to be heading for a builder/proposer split enshrined in the protocol. Until then, though, I would prefer to see an architecture that favors putting the burden on in-protocol entities and utilizes the built-in rewards and punishments system to encourage the right behavior for the good of the network.