It was released February 2022, that's only almost 4 years ago. 4-5 years is a good target for a refresh, I'll be somewhat surprised if there's not a new one in 2027 (but I was surprised by the lifespan of the Switch, and even the 7-8 years of the 360/PS3 era were surprisingly long, long generations are common now so no new Deck until 2028 or 2029 isn't out of the question), but any more frequently doesn't really make sense as the important components aren't improving in price/capability fast enough, and the initial release was and still is very capable rather than woefully inadequate. The motivations for upgrading are also different from a phone or more general laptop. I think the most common ranking of priorities for improvement would be: having various games run at all (mostly a software problem, Steam Deck already supports hardware ray tracing that various games now require), similar price range, better active battery life, physically lighter, and last would be higher graphical fidelity/performance. The things further down can't compromise the things higher up. Battery life advances being slow is kind of the killer.

There's a point that they could prioritize selling to new owners over existing owners looking to upgrade, and having a more capable device would help with that, but I think the marginal increase is probably not very big. The Steam Deck estimated sales were at 4 million units earlier this year, but that's still a relatively small portion of the whole PC gaming market (132m monthly active users on steam alone by 2021). It has been a big success for them, but it still exceeded their expectations, so I think they also would be skeptical of any large marginal improvement of new owner sales for what would likely be a minor improvement on the important specs. There's also competition from Windows handhelds whose sales don't suggest a large market just wishing Valve had a slightly more capable device that they'd pay more for.