Not from what I've seen..
I guess it's possible under certain situations, but maybe somebody else can pitch in.... Follow my logic...
You purchase 154K points at resort "A" (1 good week in a 2/2). The resort is located near attractions and is generally popular. You want to exchange your week for resort "B" and 154K pts will get you one week this year. No problem... Now, you still own at resort "A" but five years from now, all the attractions close up and the area has become a slum. I wonder if it's possible for the points at that resort to lose value?
Barring something like that, I doubt it. I have owned with Wyndham since 2000 and my first 154K is still worth 154K and still gets me good weeks when I want them....Now, over time, maint fees will go up with inflation.
But to answer your question:
"does the value of your points decrease over time so that you must purchase more points in the future to get the same # of days in the resort?"
Simple answer....No. You own the week, you own the week.
As I said, it may be possible that your week may be worth 154K points this year and 126K five years from now but you still own enough points to get into your resort for the week you purchased....