Less conspiratorial answer:
Bootloader unlock removal:
It's actually not happening all of a sudden. The dam-breaking moment is more that Samsung, the number #1 Android vendor, decided to stop supporting it.
The vendors stop maintaining bootloader-unlocking methods because the cost/benefit profile to develop/maintain/support that feature and its consequences is simply not sufficient, all while several of the biggest customers explicitly require unlock to NOT be supported.
Supporting this is not just about the unlock itself, it's about allowing this unlock (required as some carriers explicitly forbid this, so a unlock needs to be requested), then performing the procedure (using a shared secret between the device and the vendor) and then the OS continuing to boot in this untrusted state with all components gracefully handling this broken trust-chain.
The commercial incentive for this feature isn't there for a device-vendor, it actually never was. It was built, defended and fought for by passionate people (mostly within the R&D) of those companies. Companies which managed to implement it early (in times of higher product margins) were able to keep it longer, others simply couldn't get the budget to implement bootloader-unlock in the first place. Today, devices are shipped with commitments of several years of upgrades, without the vendor actually knowing yet how the OS-upgrade in 2 years will look like. Keeping his custom security-implementation is a risk-factor here
The 3rd party OS developer community was always small, and became even smaller in the past years. The footprint of alternative OS users was shrinking since Cyanogen (the leading "universal kernel" developers for Android and predecessor of LineageOS) dissolved (or tried to become a for-profit).
However, the events around Cyanogen were more of a public symptom, The main driver for people to stop using 3rd party OS's was:
1.) The increasing fragmentation of devices in the market: When the community started, the majority of the market was Samsung, Motorola, LG, Sony. Samsung was leading, but each of them had quite healthy parts of the Android market, competing with each other in an "almost-stalemate" situation. Today Samsung is leading with a huge margin, all others are basically fighting for scraps. So naturally, most of them try to go for the lowest common denominator and find a distribution channel.
2.) Android itself became more competitive: At the height of the OS community, people switched to alternative OS's to get a newer OS, new customization options and convenience features. Today, vanilla Android checks most of the convenience options already, sufficiently that most people don't want to bother researching alternative options, maintaining them, etc.
Devices of major vendors are receiving upgrades for several years (back then it was ONE major-OS Upgrade, a YEAR after Google's release, if at all)
3.) Device-integrity became more important: At the height of the OS-community, there was no Device Integrity check by Google to give a flag on whether the device can be trusted or not, so all apps kept working (with minor exception of some streaming services restricting their service/resolution, as the DRM keystore became unavailable on unlock). Today, most banking and entertainment apps rely on those Google integrity checks to decide whether they should even start. This introduced another reason for users to consider their actual need for an alternative OS.
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How to change that: If it's not possible to create a commercial incentive for the vendors, a regulatory incentive could be an option.
It's crazy to think how much computing power is just added to a drawer or landfill every day, just because there is no reason for the vendor to allow you to repurpose it.
I think this could be a path, to legally require device-vendors to provide a common SW-layer with respective documentation to utilize features of underlying hardware (optional without the shipped OS on top, disconnecting the device from the shipped ecosystem).
This would prevent e-waste and put this old hardware to better use. A community OS could then be built on top of this common SW-layer and be maintained for a wider range of devices.
I would e.g. LOVE a "Browser on everything" OS which just provides a Browser OS for outdated hardware, but the only way this could work on scale would be if the device-vendor would be mandated to provide and document the lower layer...
Someone would have to make the economic case for such a regulation as well, i.e. demonstrate the benefit for society if that is in place. The chances for this are razor-thin, especially in today's public/political climate.