The biggest problem with Windows handhelds is that modern Windows is a violently shit OS, especially for a portable. This has a self-limiting effect on the desirability of such devices. Anybody who struggled with desktop Windows will say "fuck no, not again", especially with a control scheme that's not fully compatible with a mouse-driven interface. Plus, the actually good ones (ex. Ally X) are on the expensive side.
Steam Deck is not sold on general electronics storefronts, therefore it has intrinsically reduced discoverability. Only Steam users can directly get informed of its existence. The price is circa ~$80 too high across the range. And there's still some fiddlyness involved with setting up, especially in the case of non-Steam games, or 3rd party improvements (DLSS-to-FSR, 80% charge hold).
Given all these considerable pitfalls, I'd say that the posted sales estimates are actually on the 'nearly robust' side, hence the relative optimism regarding the segment. Of course, things need to greatly improve in order to get any real traction. SteamOS needs to become fully hardware-agnostic, and get more QoL improvements (SD doesn't even have a native charge % limiter, vital on a battery-powered device), in order to spread beyond select handhelds (SD/Ally X). There are several older, now discontinued, PC handhelds (ex. AYN Loki Max) which would considerably benefit from SteamOS's ~13(+/-2)% performance increase over Windows.
I won't go too much into the Microsoft side, since I find it highly unlikely they'll delouse Windows to an acceptable enough degree as to become handheld-friendly. It would go against their whole corporate doctrine. Maybe they'll make some sort of XboxOS, but its inherent reduced compatibility with non-Xbox games will, again, limit the desirability of such a handheld (I can't see XboxOS allowing some sort of GOG/Epic sideloading, a la SteamOS). And MS doesn't have the semblance of an 'ecosystem', unlike Sony (to sell a potential Xbox portable as companion device).