I upgraded virtually everything I enjoyed on DVD to Blu-ray. Those days are behind me and I am being much more selective with UHD. I now upgrade to UHD only if it's a movie I truly love and reviews aren't terrible.

When Best Buy had the deal where you would get a $5 off blu ray coupon, I upgraded a lot of titles that were priced at $10.

If I have it on blu I won't upgrade it to 4K the only exceptions are John Carpenter movies with They Live (Studio Canal 4K,) The Thing when released in 4K and EFNY (Studio Canal 4K).
If I don't have the blu I may upgrade pending a price drop on the blu or 4K. But I don't always as it will depend on what the DVD looks like. Most still look good enough upconverted.
If I am buying it for the first time I will typically get the 4K if available.

You are wise, sir.

I have thousands of catalog BDs I won't upgrade to UHD (the BDs usually look/sound good enough IMO).

Over the years, I often upgraded my BD collection whenever a newer release with a new master came on the market.
I have an obsession with my audio and video collection being of the very best quality available...

DVDs look better on my blu-ray player than they did on my DVD/VHS combo player, so I've had very little reason to upgrade anything. The only reason I might is because the DVD went wonky (Demolition Man) or looked HORRIBLE (Outland), or I found a really good deal on a Collector's Edition BD (Mad Max) or wanted a matched set (Road Warrior/Beyond Thunderdome - actually those came in snapper cases which in my mind was a double-whammy). I might upgrade In the Mouth of Madness because it's one of my favorite horror movies that comes in a snapper case. I won't upgrade just for the hell of it; there has to be a viable reason to, I really don't want to spend that kind of money and my wife would kill me if I tried to.