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.
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.
