Thanks for laying out your concerns. As a C++ developer who does not know the other languages you speak of (I assume Rust, Go), I can agree to some of your points, but also some of them I see differently:

1. C++ can be complex, because it has a lot of features and especially the newer standards have brought some syntax that is hard to understand or read at times. However, those elements are not frequently used, or if they are, the developer will get used to them quickly & they won't make development slow. As a matter of fact, most development time should be spent on thinking about algorithms, and thinking very well before implementing them - and until implementation, the language does not matter. I do not think that language complexity leads to increased bugs per se. My biggest project is just short of 40k lines of code, and most of the bugs I produced were the classical "off by one" or missing range checks, bugs that you can just as well produce in other languages.

2. C++ no longer requires you to do manual memory management - that is what smart pointers are for, and RAII-programming.

3. I can't make a qualified comment on that, due to lack of expertise - you might be right.

4. You're somewhat repeating point 1) here with slow development. But you raise a good point: web standards have become insane in terms of quantity and interface sizes. Everyone and their dog wants to reinvent the wheel. That in itself requires a very large team to support I would say. As stated for point 1), I do not agree development in C++ has to be slower

5. True, as someone who just suffered from problems introduced on windows (cygwin POSIX message queues implementation got broken by Win10, and inotify does not work on Windows Subsystem for Linux) I can confirm that while the C++ standard library is not much of a problem, the moment you interface with the host OS, you leave the standard realm and it becomes "zombieland". Also, for some reason, the realtime library implementation on MacOS is different, breaking some very simple time-based functions. So yeah, that's annoying to circumvent, but can be done by creating platform specific wrapper libraries that create a uniform API. For other languages, it appears this is done by the compilers, which is probably better - meaning the I/O operations got taken into those language's core features

6. I am highly doubtful of people relying on garbage collection - a programmer that doesn't know exactly when his objects come into existence, and when they cease to exist is likely to make much bigger mistakes and produce very inefficient code. The aforementioned smart pointers in C++ solve this issue: object lifetime is the scope of the smart pointer declaration, and for shared pointers, object lifetime expires when the last process using it leaves the scope in which it is declared. For concurrent programming, I do not know if you mean concurrency (threads) or multiple people working on the same project. While multi-threading can be a bit "weird" at first, you have a lot of control over shared variables and memory barriers in C++ that might enable a team to produce a browser that is much faster, which I believe is a core requirement towards modern browsers

As for your tl;dr: definitely not "less concurrency", that makes no sense. The other points may or may not be true, keeping in mind the answers I gave above.