I'll still buy discounted Disney GCs regardless of my UR decision - so I've already factored in that the $634.32 upgrade cost is really only $588.20. My thing is that I don't need to redeem to offset this (and I realize that's specific to my situation and may differ from how others look at it)...I just needed to make a decision on whether I wanted to spend $588.20 or not. I think we all just have to figure out how we choose to mentally offset points and savings vs. travel costs.
I think it's hard for me put my thoughts into words, but basically: If I want to cash out, I can...if I don't want to, I won't...but cashing out wasn't going to affect my decision on this upgrade. That was a decision on whether it was worth $588.20 to me (and I decided this morning that it was).
With regards to Ultimate Rewards Points dropping in value...Chase Ultimate Rewards points being worth $0.01 in cash is the primary tenet of their credit card rewards strategy and marketing. It's what makes the Freedom a 1% back everyday card and what makes CFU a 1.5% cash back card (not to mention CIC and CIU on the biz side). While fear of devaluation is a real thing, that typically is related to hotel and flight points and not cash value of major issuers points currency. Chase URs have been been worth $0.01 each in cash since they were introduced roughly a decade ago (I don't have actual stats on this - just sourcing my memory on card intro dates).
I think devaluations in future earning could occur on Chase UR cards, I think devaluations of the 1.25 and 1.50 redemption options could occur, I think transfer partners could be eliminated (and they could abandon the 1:1 transfer concept however that's also been a mainstay of the program).
All that said...I personally believe that a devaluation of URs to be worth less than $0.01 each in cash will never occur. I have no insider knowledge on that and I may be proven wrong someday but it is a risk I am happy to take for now. I would much rather have the opportunity in the future to use those points for $9,000 than have $6,000 in the bank today. Of course if this was a trip that was impairing our current financial situation I would probably be redeeming those URs (but it should also be said that if this trip were enough to impair my financial situation - we would be cancelling it, lol!)
ETA: I'm not suggesting what I say above is the "RIGHT" way to think about things...I don't want it to come off that way. It's just the way I choose to look at it.