I've had a rather bad week, the kind of week that leaves one looking for a pressure valve. Reading the other post you linked to in a comment, insulting the huge body of work given to you for free, making repeat posts on it... I'm going to go a bit out-of-character, let go, open the valve, and ask;
Are you a troll? Are you a moron? Both? Did you create that account solely for the purpose of covering the everlasting shame that is the words you have typed on the internet?
"after 20 years [...] after 20 years [...] after 20 years" KDE, as a "thing", 1.0, was released 17 years ago. I seriously doubt you'd be a 1.0 user to know what the first release looked like, I'm not. Half of what you complain about is neither KDE specific or even 20 years old.
Mail client is buggy? Are you aware that every email client has bugs? That you can't escape them? Did you know email as a protocol is so hopelessly broken writing a client for it is as simple as interpreting abstract art? Apple Mail: certain IMAP conditions will cause send/receive to completely break. Thunderbird: currently not getting a mail I know I should have received - had to use webmail today. Bugs! They exist! Kmail, I'm probably going back to it because it worked, only moved off because I wanted in-email GIF animations to mess with my co-workers.
FTP doesn't exist? It does, in Dolphin. For reasonable use. But you have 1500 accounts, a frankly stupid number that tells me you're mismanaging something. I mean, if you actually need 1500 accounts you are doing it wrong. The only justification is having 1500 clients on 1500 different providers, and in that situation you would have to be an idgit to be putting 1500 sets of access information onto multiple machines. Lose a machine? Guess what, you've just compromised 1500 systems. You couldn't reset those credentials fast enough. You either don't know how to properly commission servers, or are a single horrible point of failure for a huge number of systems.
Printer complaints? That's CUPS, CUPS is just under 16 years old, and the talented people at Apple you revere are the force behind it. For scanners, 90% of it is SANE, and that's 19 years old, released before KDE ever existed. Neither are KDE specific.
Plasma is buggy as hell? I've commented on this too many times to be courteous to the troll; check your drivers, check your versions, and use semi-modern graphics silicon. Plasma will make fuller use of your hardware capabilities, and if you don't have the hardware or the driver support it would suck just has hard as trying to run Windows on a toaster. Even then, plasma 5 is running on two ancient business machines from last decade without a problem.
Multiple cursors in KDevelop? How bloody specific are your complaints going to get? And they already implemented it!
Just... I'm going to be honest. Maybe it's because I'm in a not-so-great place right now, maybe it's because you've pressed all the right buttons, but I'll tell you something I've learned:
When someone has problems with everything it's not because everything is problematic. It's called PEBKAC, or "Problem Between Keyboard and Chair". Instead of preening your feathers and romping around reddit claiming to be an "FTP Power User" (made me laugh, I want to put "FTP POWER USER" on a T-Shirt) you should probably sit down and learn what you're doing before insulting everyone.