It seems that they did not successfully attract most of old professional devs to embrace the new platform. Java was already famous (and infamous) enough to get more noob devs at that time.

In short, it seems that you don't have ever make any cross-platform framework that just work. Surely any developers working with these "mobile platforms" should have known how shitty the differences they have to work around among these systems (and those who maintain cross-platform frameworks should know more), it does not mean that there is nothing to do and the platform-specific solutions will definitely work better. That said, Dropbox may not be totally wrong, but it's things about hiring what kind of programmers rather than maintaining what kind of codebases.

Not quite true. They are not good at designing a solution (of such applications) sanely, where all resources are identified having proper ownership before any coding works. In short, "life cycle management" is not the work that most application developers should ever do. Such things should have been specified in the design independent to languages to implement it. The complexity for coders is not the excuse to ignore the complexity of the solution. And one sad thing is historically few users have respected to such rules of sane design/implementation separation, except users of idiomatic C++. It is getting better when more modern languages (e.g. Rust) do it right, but C++ is still almost the most prominent example in the industry. "Not good at C++" is not so strange in this aspect.