I'm afraid SDK v1 addons cause catastrophic compatibility problems and the risk of keeping them is unacceptable. Due to the lack of encapsulation, addon developers have written lots of addons that manipulate the internal details of the engine that were never meant to be used. If I merely rename something or move something around to better organise our code, it can break addons, causing problems for hundreds of customer projects, who then all contact support and blame us for breaking their project in the latest release. This has, and will, happen over and over again. In the long term all parts of the engine eventually change, and so eventually all such addons will probably break. It will leave us in the position of having to either stop making improvements to Construct, or break lots of projects.
This is thoroughly understood stuff in the software industry. For these reasons no other professional tools allow unrestricted access to internal engine. The industry standard for decades has been to use encapsulation and enforce that only a specific documented API can be used. Then you can ensure everything keeps working in the long term, even when doing major internal upgrades, and even after the addon developer left the community many years ago. The only reason Construct ended up with a lack of encapsulation is historical reasons - basically JavaScript did not used to support it.
I know this will be a painful transition but unfortunately I believe it is absolutely necessary to make Construct reliable and trustworthy in the long term. The next LTS release will be supported for another 18 months so there is still a long period of time that SDKv1 addons will still be continued to be supported in LTS.