I've been in the Android scene a little longer already. Maybe not as long as some others, but Android 4.0 was the first version I used. A lot of time has passed since then. But when I look back in time today, it strikes me that one thing that had found many followers at the time is no longer really important.
Where are the Custom ROMs gone?
When I got my first smartphone, I quickly realized what custom ROMs are and what they're good for, but only after a while when I found out that I can root my phone without unlocking the bootloader and without losing my guarantee, I decided to try it myself.
For the mobile phone, which my parents gave me - it was a device quite far in the budget sector and not so widespread, there were at least 10 different custom ROMs, which you could install without unlocked bootloaders.
For the mobile phone I use today - a 2015 Moto G from 2015 - there are not really many usable alternatives besides LineageOS. And the Moto G is much more popular than my smartphone at that time.
Who's fault is it?
I think there are several reasons why custom ROMs don't find the appeal they used to have:
On the one hand, it is likely because the manufacturers are making it more and more difficult for power users to unlock the bootloader, so that it is even possible to install another operating system. If manufacturers don't voluntarily point out ways to unlock the device in the normal way, security vulnerabilities must be found to make the whole thing happen. And security vulnerabilities of this kind can also pose risks, which is why many manufacturers are interested in eliminating them.
Another reason is probably that there are also a lot of devices that come with Vanilla Android, i. e. without pre-installed manufacturer UI, which also comes with a lot of bloatware. Since many people used to want to get rid of these apps and all the storage space they wasted, they installed custom ROMs that gave them a better user experience.
A third reason: Even though Android fragmentation is still a problem today, it used to be even worse. Many devices have never seen an update after sale and many users were just not willing to spend that much money on a new smartphone just to get a newer version of the operating system. Even though I sometimes wonder if devices like Google Pixel are too expensive, there are guaranteed updates for a few years. And for many, the price is simply worth it.
Do you still use a custom ROM?