I've certainly accumulated far too many valves, for sure!! Some of them have been very good investments, like the DHTs. Some of the more uncommon types will be hard to shift, good as they all are. I bought a lot of European tubes before Brexit, and that was a wise move. Tubes like EL12 and EL11 which have a following in Europe, but little known over here. And some less common types like EL41 and E80L. All brilliant sounding valves. And within the UK I've been collecting EL33 and PL33 (got 25 of those). EL33 is a particular favourite - beautiful sounding valve, very like EL11. Customs and VAT charges now make European vendors uneconomic, which is a real shame since I love the European valves more than the US valves. The German Post valves are legends in Europe.
But I digress...... I don't use any of the common types like KT88, 12AX7 and 12AU7. If you collect these common valve types in an intelligent way your investment may be fairly safe. I used to buy the US types in a big box to make the shipping economic - the US types are cheaper over there and if you know your dealers you can get good prices for tested valves. Back in the day I had an "insiders" list of small dealers who still had stocks of desirable valves they were willing to sell cheaper than the big dealers. But they've long since been cleaned out by collectors. I myself cleaned them out of a lot of 01A, 26, 46 and 10Y. But you can still get good deals on the common valves. The RCA types are usually very good, but they all can be collectible.
So collecting NOS or good tested (80% plus) US valves is still a potentially decent investment. Prices have remained quite stable since the pandemic, maybe because people in lockdown are paying attention to their sound systems. The days of massive profits, however, are gone. Triple mica 5751s are a good example - silly prices now.