Because unless we remove it, people will just carry on using it, and the disasters will still happen
Well if there is a clear warning, the user would know they're taking the responsibility, if they complain to you, you could just copy paste the link to the documentation page telling "If you use a Addon SDK 1 addon, it could be broken by accident and we won't provide support". You could make it totally inaccessible for child/student by restricting Education Licence and hide the SDK 1 compatibility behind the hidden developer mode setting, so only the advanced users that actually want/need to extend the engine would do it.
Before the warning was adressed to addon developers, but now the warning would be addressed to users of the addons, which is far better.
Also hiding risky features behind a Developer Mode is an "industry standard" everyone understands.
There is no valid reason benefiting any C3 user to justify the fact to permentently remove so much possibilities and control over our own work, the only person on earth that think they might benefit from this situation is you. And we're several to think you're actually wrong and you're hurting your product/community by doing so, this just is a lose-lose situation while every single competitor out there understands how a prolific 3rd party dev community is a win-win scenario.
Disasters have happened, are happening, and will continue to get worse.
I get that "disasters have happened" at the C2 era or 5+ years ago when everything was different.
But the "disasters ARE happening" is just false, since worker mode support introduction (4-5 years ago) that involved some changes on how addons should be made, the vast majority of addons made never broke. Also the very few time there was disruptive change like that, there were clear benefits for all users in the balance. (it is the opposite here, you dedicate months/years of work to LOCK the engine instead of enhancing it and acting on features requested for years)
I'm not sure if there is a single Construct 3 addon made over the past 6 years that would be impossible to fix in the current version thanks to SDK1 even if it broke 3 years ago. Currently there is always a workaround, or a simple fix to find. SDK2 is the guarantee there would be no workaround at all in a tremendous amount of situations
Construct 3 runtime codebase never changed on many aspects, because all vanilla Plugins and Behaviors (or any other features) are relying on it and you've always been reluctant to make even the tiniest change to those official Addons to make sure they still behave exactly the same. (so the reason it's so stable never was to avoid breaking 3rd party addons but to make sure games using official addons still do the exact same thing)
Even when something changes (if it ever changes), it's very quick and easy to fix, like just renaming a method or rewriting 1 or 2 lines do the trick, because, to keep working, the C3 codebase still need in some way the same feature we were accessing.
It's nothing like having to rewrite EVERY addon from scratch with a 50% chance it won't ever be portable at all.