a) pretty much any closed source project or online service can be done just as well by gluing together parts of open source projects…
Which may be true in some cases, but the real misapprehension is the final part:
b) …and it won't be difficult to do.
The classic is the HN comment about Dropbox, but I see at least one comment a week saying something similar. No doubt there are many more.
Outside of very specialized tools, open source tools tend to attract more contributors and quickly overtake incumbents (see compilers). JetBrains is one of the few companies that bucks the trend, and one I gladly give money to regardless of my increasing VSCode usage.
Had Dropbox been marketed at developers only, it would have been a spectacular failure in a world where inotify, rsync, cron and scp already existed.
Large parts of IntelliJ are open-source as well, and sometimes they do get used in other open-source editors. Afaict they're pretty good citizens in terms of F/OSS.
The optimism bias is a problem even for programmers.