@ spuddybean
Personally i wouldn't want to be with someone who didn't want to share their money with me.
which is interesting because a few days ago there was a thread in which a poster who said she had won a few million on the lottery was advised by many not to marry - so as to be sure to keep her money. whether the post was genuine or not, it was an interesting series of responses.
the issue the OP's SO probably has is that he's been divorced once and paid up, and then broken up from an unmarried relationship and not paid up. it's zero sum of course, his gain was her loss, but his strategy is working for him.
he clearly has plenty of appetite for LTRs having spent 35 years in them, but both ran their course and both ended. if he'd married his second LTP it could have staved off the separation but there's no reason to assume another cause wouldn't have arisen by now.
IMO the OP now needs to convince him that there's something in it for him and that this relationship, unlike the last two, is for keeps. if he goes along with this and is wrong, given that his working life is over he has no realistic prospect of regenerating any wealth of his she ends up with.
prenups aren't the answer because their legal worth is questionable, especially if circumstances change between times.
it's an interesting question really. if you could get married only on the legally enforceable condition that you waived your right to half the joint assets in the event of a divorce, and got back out only what you had put in, would you still do it?
IMO the OP should identify 3 or 4 benefits to him of getting married to her. "If you marry me I won't leave" probably shouldn't be one of them, because it appears someone tried that and it didn't work.
if you can't think of any reasons that benefit him, that's your answer i'm afraid.