The only models I've ever regretted buying were ones based around some rule I thought was cool instead of the rule of
cool, and those ones usually have decent resale value anyway (I'm looking at you, vanilla Hammernators). I sell stuff that I can replace with cooler new models (like the new BA Tacs and Termies) but keep pretty much everything else. I also have about 5k in models that is basically auxiliary to my main BA collection and frequently sell those models (that I got in sweet deals as add-ons mostly) to fund the newest shiny GW's come up with.
Did I play my BA Tacticals or Death Company in 5th edition? Never. Do I use them in every single game I play now? Absolutely. Shit, I even started using Dante again and I haven't used him since my first game of 5th around 5 years ago, and before that it was likely 3rd edition. I've owned that model for around 20 years!
Do I keep them, locked away in hopes of them someday getting overhauled?
At least put them on display.
Do I build an army list out of the Necron Codex and proxy the army with those models?
Absolutely. Granted, I could play a 'better' BA army out of the vanilla 'dex...but why the hell would I? Unless you're at a tournament winning shouldn't be the
reason you're playing the game. Have fun! Play against people who feel the same!
Or do I sell the whole thing, and if it ever goes Plastic, start fresh?
If you love the army (fluff and models you own), keep it. If you put effort into the models and you can see the history of your hobby in that army, keep it. If it goes to plasic, incorporate the plastic into your army. If after you've done that you strongly dislike the old models: sell them to buy more fancy new plastic!