Ever regretted starting a new file?

I started a new run on my Platinum version as a kid way back when it was the newest game and regretted it massively not long after - most of the Pokémon I've been transferring up from past gens were there, including some cool shinies. I wanted to play through Sinnoh again really bad but still wish I didn't do that and lose everything I'd collected. If I could go back and undo that, I would.

Have you had any instances of making a new save file and later regretting it?

Yes, but only much later. Dug out Yellow recently. Hoped to see the team used. Must have made a new file to replay a Generation 1 game as a child. (Sunk a lot of time into Red. Deleted the one with less attachment.) Gleaned nothing interesting from the new Yellow file. Consisted of level 15s or so.

Regretted none besides that so far. Never deleted any files in Generation 2-5, though. Hated to lose them. Includes even the glitchy Diamond file. Only began doing so again with the ability to transfer Pokemon off and return them, if so desired.

The only save files I deleted to restart the game from scratch were on the 3DS, and that was after I tossed anything I wanted to keep into Bank.
Edit: seems I forgot about my Ruby and Sapphire resets at the time of posting this lol... answer is still the same though.
So, no.

Yup, deleted my Diamond save - with Pokemon dating back to some of my early Emerald saves (lot of restarts but probably still 2006), and LeafGreen which included a shiny Dragonite iirc - for a trip to metro centre…where my mum bought me Platinum.

Also selling back SoulSilver for HeartGold instead of spending money - cost me a ton to get it this year although I lucked out and purchased ones with unused Enigma Stone events and Yellow Forest on the Pokéwalker!

Finally, restarting Black in 2014. I smashed the game fast enough to get a redo and still download Victini…not the case the second time…and I didn't even like Serperior! Went out and bought a copy of Black 2 that summer as well so if I held out could have transferred my Pokémon (including Dream World 'mons and berries). I came with a movie event Victini at least but it's not the same.

Diamond is definitely the big one though.

Not at all. Whenever I felt the need to start a new game, it was for a good reason, no doubt.

it should probably say something that every pokemon game i currently own i don't plan on ever really doing a fresh run on. i've spent way too much time on those games to justify starting over, not that i would have the time these days anyway to progress at any significant point compared to back then.

Oh yeah, definitely regretted deleting my Platinum save file... same for my original Soul Silver save file. & my first Emerald playthrough. I can never get those back. I had some Pokemon I enjoyed on those games that I'll never see again if I didn't transfer them over...because there's plenty I did not, I'm sure.

Not particularly no, I never really cared much for collecting or interactivity so after I was done with the main campaign the save file could safely be deleted.

I usually transfer any Pokemon i cared about over to a different game, i.e. Poke Transfer Lab for gen 4 pokemon, so that way i'm not losing anything important. And some emulators have the ability to toggle different save files on the fly, so i see that as an absolute win!

Nah, I don't have the interest or patience in playing Pokémon games with a completionist mindset, and most of the time I just drop it right or shortly after the credits unless there's a very interesting post-game (which is normally not the case) so I don't really have anything big to lose.

I'm sometimes hesitant because I know I'll lose my former team, but these late years I got the habit of recording key moments of my playthroughs to keep memories of my previous teams easily.

I'm hopeless since my favorite part of any game usually is the beginning haha. Especially in pokémon. I've restarted my pokémon games too many times to count, even when I had collected nice pokémon and items. I'm just not a completionist or a collector in that sense, and am really just in it for the journey. So no, I frequently restart with no regrets.

Back in the Gen 4-5 days, yes. I had no way to back up my saves or the Pokémon on them.

Since the advent of Pokémon Bank, though, I do everything in my power to preserve my old Pokémon. I'm still not resetting my save in Pokémon Sword or Brilliant Diamond because there are so many event items I would lose if I reset them (particularly the latter, because I can't get the Platinum outfit back if I were to reset). I circumvent that by starting new files on different Switch profiles.

When I got into Silver I mass traded all the Pokemon I favored from Yellow and re-started it more than once.

Recently I have started to regret that a bit because somehow my frickin' 20 year-old Yellow save battery has outlived all my other generation one cartridges and still lives last I checked so had I not done that trade and overwrote that first file I'd still have a lot of the Pokemon I lost when my Silver battery died such as my Pigeot Windy, the first Pokemon I ever caught and raised to level 100 in Silver.

I know I could have still saved them using Stadium, I did once own them both but back then I was just learning about save batteries so wasn't even considering the possibility. I didn't know this then but my Silver had actually lived 2 years past gen 2's general lifespan of 5 years.