Depends on the cost of energy and the interior comfort level you want.
1500 is not much if you compare it with any tech device upgrade cycle.
The seal between the pane and the wood is usually glazing. If that’s failing, you need to replace it. You can buy glazing putty at a big box store, and see how to do it on YouTube. With a little practice, it’s about ten minutes/window. $15 in materials will do a dozen or so.
If it’s leaking between the wood, you need to caulk. That’s $3-$13 in materials/tools (depending on if you already have the gun) and a couple minutes labor.
You may also need some flashing over the top to direct the water away from the pane. This is slightly more complicated. Probably $30 and an hour or so.
If you don’t want to learn these skills, a handyman should be up to any of them.
Multi pane windows also need to be flashed and caulked, just like your single pane. The only piece that I mentioned that is different is the glazing. Glazing can be replaced, the interior seal on a multi-pane window cannot
... however, I do prefer the modern ones, myself. Energy loss through single-pane windows is no joke.
FYI, you can usually replace just the double-paned glass insert in those for a small fraction of that cost.
Some of the double panes added during the 90s remodel have busted seals and need to be replaced.
It’s a minimum $1500 to get a window replaced around here with a quality modern replacement. Assuming a generous 50 year life, that’s still $30/window/year for a replacement, plus the opportunity cost of not investing that money. Economically it almost certainly does not pencil out to replace the original windows.
There are quality of life benefits as you mention, but I can understand how someone who likes the aesthetics of old single-pane wood windows would choose not to replace them.