There is no way for Disney to force OLC to spend money. If OLC agrees to a re-theme, it's because they truly believe they will make a superior product. And I have had no reason to doubt OLC yet. Plus - OLC has already turned down a Splash retheme once before.
I'm sure this is among several decisions they're considering for how to improve Tokyo Disneyland next (what a blessing to have a company only focused on tourism!). And regardless, there is absolutely no way this is happening before Space Mountain opens in 2027.
I have no doubt that one day Tokyo Disneyland will replace Splash Mountain. But that's only because they continue to pour money into making their park newer and better. I have complete trust in their management. And honestly - I wholeheartedly support their decision to continue to make Tokyo Disneyland a completely unique castle park instead of the magic kingdom imitation it began as.
In the meantime, I look forward to riding Splash Mountain on my next trip!