that in 2026 it will not be able to be updated
Actually maybe
Ashley could clarify this but I thought that r449 LTS would stay available beyond 2026, just not receive any more updates. Similar to how all versions of Construct are still available. I can open r100 just fine still if I want to or need to.
Either way, you may have to do a version freeze. Stay on r449-2 which is the final version that supports SDK1. It's not unusual practice to do in development, especially when you reach later stages of the project where updating the engine could introduce a million random regressions and bugs.
we have many times hired services to make custom addons and now all those efforts are useless
Apart from using them in the LTS version, you can still update these addons to SDK2. You should be able update any addon that doesn't hack any engine internals without much issue.