OK, I do not have any numbers to back up my statement, so my impression could definitely be wrong. And I definitely agree that the number of users is going to increase, with 5G fixed wireless access pretty much replacing DSL over the next few years.
Sure. I could not agree more.
And I do understand that modem firmware upgrade might be a required feature. The reason I consider the number of users limited wrt that is because it's a much more hardware specific feature than the rest of the modem management. MBIM and QMI hides most of the differences between modems, making an old D-Link 3G modem behave pretty much the same as a modern Quectel 5G modem. I believe this is one of our major success stories. Qualcomm always have pushed a new driver per modem with minor differences. It was Microsoft who forced them to accept MBIM. But we managed to make the same qmi_wwan driver support all of the Qualcomm modems. And that's not just across vendors, but also across chip generations with support for anything from the 1st generation Gobi1k of 2007 to the most powerful 5G modems of 2020.
But if you are going to support firmware upgrades, then you don't have this advantage. Every modem is different. And it's not just about different firmwares. All vendors user their own scheme. A vendor like Sierra Wireless has changed upgrade protocols 3 times over their last 4 modem generations. The number of OpenWrt users running the same firmware upgrade code is going to be low for most modems. This implies very limited testing. Given the possible errors, the risk analysis is not looking good.
Note that although I have written qmi_wwan and cdc_mbim and added support for the gazillion different modems they support, I still don't own more than 10 (or so) modems myself. And I'm only ever upgraded one of them on OpenWrt and 3 different modems in total on Linux. I definitely do not feel like supporting firmware upgrades in general.
As for PUK code entry, that is a procedure shared between all the modems. The reason I still think it's hard to get proper exposure is that it's not something you ever need to do. When you configure an interface with a PIN enabled SIM, then you either enter the correct code or you don't. If you enter the wrong code, then you fix that. Then you don't change that interface configuration again. It's very hard to get yourself into a situation where you would need the PUK code on OpenWrt. And even if it happens, it's probably easier to temporarily move the SIM to a phone for recovery instead of figuring out where to put the PUK code in Luci. So my estimate is that you are lucky if you get one real user/tester of the feature every 10 years or so. And you will probably get 200 users every day wondering what that PUK setting is supposed to be.