Obviously different standards of reasonableness have been expressed in this thread.
And hopefully all the historical background, pros, and cons of the rule have been explored.
Mike, you continue to focus on defining more clearly reasonableness. You seem to be discounting the fact that the rule also changed how a bet that can reasonably have multiple meanings is ultimately ruled.
Don't think I've avoided that fact, just trying to explain the history of the rule and how it evolved to the present wording.
There are actually 3 important issues: legality of a bet, reasonableness of a bet, and consistency of ruling. And seen that way it's pretty simple:
1: Legality. NLHE 250-500. 18,000 in the pot. Player declares "five". Both 500 and 5,000 are legal bets.
2: Reasonableness: The 2013 and 2015 instructed TDs to rule the lesser of, 500 in this case, unless it wasn't a reasonable bet under the circumstances. TDs were clearly not all on the same page as to how to interpret reasonableness. Some thought 500 unreasonably low here, some thought it "low but legal" while others just always ruled "the lesser of". Reason: "reasonableness" was left in the 2013 and 2015 language to be at TDs discretion.
3: Consistency: As discussed in #2 above, the 2013 and 2015 language resulted in inconsistent rulings.
SO, options:
1: Always the lesser. This is consistent, it's legal, but it reaches a point for a super-majority of Summit attendees where it just isn't reasonable.
2: Leave it up to the TD: This is legal, it's reasonable, but it isn't consistent.
3: Adopt a reasonableness benchmark such as "the highest amount that's covered by the pot". In the example, an 18,000 pot covers a 5,000 bet, It's at better than 3 to 1 odds, and is the bet size most likely to be made the vast majority of the time. It's legal, it's consistent, and it's reasonable. Keep in mind it was proposed in 2015 and has been trialed at the WSOP for some time without major problems. Of course it will be re-visited in 2019.