Yes, there has been a big increase in the time that Android/AOSP phones can be supported since Qualcomm and Google made this announcement in Dec. 2020:
Google and Qualcomm are teaming up to enable a longer support window for flagship Android smartphones. Qualcomm, with Google’s help, will now support its chipsets for three years of major OS updates and four years of security updates, enabling a better-than-Pixel level for all future Android phones, provided your OEM is willing to cooperate. This policy is starting with the flagship Snapdragon 888, but even lower-end chips will be supported.
However, what is not being promised is any upgrades of the kernel for Qualcomm processors, so that puts a hard limit on how long Android/AOSP phones can be supported. Samsung made a similar announcement in Aug. 2020 that its flagship models (S/Note/Z) starting with the Note 20 and Z Fold 2 would get 3 years of Android upgrades.
You may get lucky with a phone like the LG G2 because the LineageOS maintainers figured out how to keep upgrading AOSP, but that generally doesn’t happen with most phone models. (I don’t know if they figured out how to upgrade the kernel on the G2 or just run new AOSP versions on a really old kernel in the G2.)
Google provides security updates for two years for each version of Android, so if the phone OEM (or the carrier) decides to not offer Android upgrades, the phone is effectively limited to a 2 year lifespan, and many phones, especially mid-range and lower-end models aren’t getting Android upgrades.
However, even if you have a good phone manufacture or you have a model where you can install AOSP on your own, the lifespan of the phone will still be limited because the major mobile SoC manufacturers (Qualcomm, Samsung, MediaTek, UNISOC, Huawei/HiSilicon and Google) generally don’t support new Linux kernel versions for their processors, so you are stuck with the same kernel for the life of the phone.
Each version of Android/AOSP only supports 3 LTS kernels. For example Android 12 supports Linux 4.19, 5.4 and 5.10. If you buy a phone like the Google Pixel 6, you will get Android 13 and 14 upgrades in the future, but you will still be using the Linux 5.10 kernel, and Linux 5.10 won’t be supported in Android/AOSP 15, so no further upgrading is possible.
Fairphone 4 (FP4) is doing something special because Fairphone is promising that it be supported by postmarketOS and Fairphone is saying that the phone will be supported by mainline Linux, so it won’t be limited to just Android 11->12->13 like a normal phone. Maybe Fairphone secured a promise from Qualcomm that it will offer kernel upgrades for the Snapdragon 750G or maybe Fairphone plans to compile its own kernel so it is possible to upgrade to Android 14 in the FP4. Given the problems with the FP2 and its support ending after 4 years, I await to see how Fairphone supports the FP4 till 2027, but I give Fairphone a lot of credit for collaborating with /e/ and postmarketOS, so there are AOSP and Linux options for the FF4.
Modularity in a phone only makes sense if modularizing components which are liable to break or can be upgraded–otherwise modularity only adds extra to the environmental and economic cost of the phone. With the Fairphone 2/3, there was a module to upgrade the camera, and the USB port and 3.5mm audio jack were modularized, which is important because they are liable to break, but I don’t see much utility in the other modules in the Fairphone. The FP4 gets rid of the 3.5mm audio jack (which makes most people’s headphones obsolete), and I doubt that Fairphone will offer a camera upgrade module for the FP4, since there isn’t much point to having over 48MP of camera resolution, even if the Snapdragon 750G can support up to 192MP.
The L5 has modularity in the battery, USB-C port (on a separate PCB), WiFi/BT, cellular modem. Being able to change the cellular modem allows you move to new regions of the world and support new LTE bands, which is not possible with the FP4 which is limited to just European bands and has the modem incorporated into the SoC, so it can’t be changed.
If you live in Europe, I think that the FP4 is the best Android phone that you can buy, because it will be supported by mainline Linux and postmarketOS to escape planned obsolescence, and it has good enough hardware to be a useful device for a long time. However, buying the FP4 won’t help pay for dev work on mobile Linux like buying the L5. If the goal is long-term change of the mobile industry and trying to tackle the problems of planned obsolescence and Surveillance Capitalism, there are good reasons to support the development of Linux, whereas AOSP isn’t a good vehicle for promoting change in my opinion.