The blocks with the "sactioned" transactions will be ignored by most of the people, so they will only live in forks that everyone will ignore.
Assuming enough hash rate and the exchanges (and the users) agree to ignore the "sactioned" transactions, then they will never be in the main chain, not even slowly.
Which one is the "main" chain is entirely/ subjective. If users prefer the unsanctioned chain, it can very well be "main", regardless (see BCH/BSV debacle on "who's the real BTC", also ETC).
Also just noting that while the conversation here is about Ethereum, you are describing the fork rules of Bitcoin. Next month the concept "hash rate" won't exist on ETH mainnet (potentially depending on who you ask; looks like there might be an ETC2).
If that's what it comes down to (and there is reason to believe it won't), I imagine sanction addresses will still transact, just with higher latency and at higher cost than others.