I'm a big-time redstone/command person. I've been programming stuff with vanilla commands for years now. Yet it seems like every time there's a big update, like with 1.13 coming out soon, there are always some big changes made to existing command syntax that makes it so I have to go back and "fix" my redstone creations every single time.
Look, I love it when new commands/command abilities are added. The new target selector argument "nbt" is something I've been asking for for ages now. But there are other things that I have a slight issue with. The new /data command seems fine, but did you really have to remove /entiydata and /blockdata? I know /data might have made them obsolete, but mapmakers, programmers, etc will now have to go and change entire aspects of their redstone just so their creations don't completely fall apart once the update comes out. It's not just /data - the scoreboard target selectors were completely revamped, and with really big command creations, those target selectors are often present in at least half of the command blocks. And did you really have to change "c=" to "limit="? And "m=" to "gamemode="? That seems totally pointless imo.
This isn't the first time, either - it gave me a total headache when the "Equipment" tag was changed to "HandItems" and "ArmorItems". But you get the idea. I guess what I'm asking here is that you try to eliminate this issue as much as possible, by letting previous command syntax still be viable. If you really must change it to something more "easy to work with", then go ahead, but still allow the previous method to still work. Like, sure, go ahead and add /data, but don't remove blockdata and entitydata because of it. I'm sure I speak for more than just myself when I say that it's frustrating to have to constantly update my creations to prevent them from breaking.
I mean, if they're going to completely revamp the commands and not have backwards compatibility (BC), then they could have at least provided a tool that goes through your maps and updates your commands for you. But yeah, I definitely think there should be BC. Imagine if Java one day just decided to completely change, with no BC. There would certainly be a riot among developers who now have to completely relearn and rewrite their programs or stay on an unsupported version.
110% Support. Unfortunately, knowing Mojang, I doubt they'd do this.