Older telescopes can still contribute to science in parallel, too. Even terrestrial telescope time is booked years in advance. More scopes means more time for secondary pursuits, hands-on time for more junior astronomers, observations for more speculative theories, etc.

Terrestrial telescopes don't have nearly the degree of maintenance costs that a manned servicing mission in orbit entails.

If fixing your old terrestrial 0.5m telescope were going to cost more than a shiny new PlaneWave 0.5m, you can bet that you're going to replace.

Of course, the cost tradeoff is quite different. All I'm saying is that a telescope, ground or space, does not suddenly become irrelevant because it's not the latest and greatest, so the expected value of the Hubble may remain positive after newer scopes are launched.

Each new space based platform also is built to see different parts of the spectrum. Hubble happens to be mainly visible. It is not irrelevant just because JWST is now active. To the contrary, they are combining images from each platform as a composite because they see different things for the same objects.

There would be no purpose of launching a new visible spectrum space platform unless it is going to seriously dwarf the size of Hubble. Hubble can already be upgraded with new sensors, and has. Which is part of the reason Hubble remains relevant. Hindsight being 20/20 and all, JWST learned from Hubble's need for contact lenses, which would be assumed to follow for whatever next is.

Because servicing it (beyond reboost) may be the same cost as launching something new that offers better performance.

The proposed "better Hubble" that would launch in the 2040s is projected to cost $11 billion. That's a fair few servicing missions; it sounds like we can probably afford to do both, and if so, why not? Keeping Hubble around doing science longer is still useful even with a successor planned.

https://www.nytimes.com/2021/11/04/science/astronomy-decadal...

Since Hubble is at the upper range of LEO which made shuttle missions difficult, I've always wondered if they could make it slow down enough to lower its orbit more easily accessible for orbiters where part of the refurb is to refuel it so that it could get back to operating orbit.

Maybe not Hubble, but maybe the next gen?

when it comes to something like this you are forgetting the government redtape involved in building a new one is likely a bigger cost than the actual engineering building and launching.

Not to mention that NASA doesn't have the kind of blank check budget that the military has (by a longshot) — in a budget overview they may have to cut service for old projects to even get research on new projects funded.