I use an upright freezer with my Stout 14.5 gallon conical.
The only real issues I have with a plastic conical are that you know it will need to be replaced one day... and they really don't have the resale value when you want to get rid of them. I've seen this guy on craigslist trying to sell 2 of them for 3 years lol.
Do they really *have* to be replaced? Even if they get scratched up a little on the inside it still seems like a long oxy or PBW soak followed by a long bleach soak would clean out anything that might be trying to shack up in there.
Alternatively, any reason why they could not be put to use brewing sours?
I suppose... I know some of my old bucket fermenters have absorbed alot of interesting aromas and flavors that even long PBW soaks will no longer remove. Especially if you happen to do a really strong flavored melomel or metheglin in them etc... I have a bucket that to this day still imparts a subtile spearmint flavor to anything fermented in it since I used it for a spearmint mead...
Dedicating it to sours I suppose could be an option. Though I still am very sanitary with my sour beers. There are lots of wild yeast that do not impart what I would call 'good' flavors/aromas.
I'm not saying that a plastic conical can't last you many many many batches without being replaced, if properly taken care of. Obviously there are different manufacturers of plastic conicals out there (I own one of those Onederbrew ones that fits onto a bucket) and they have many different price points.... But for not much more of an added cost you can get a stainless conical which will retain value better (if you ever decide to sell it) and it will be easier to maintain.
Just a quick look a mini-brew 8 gallon plastic conical is $349 and a 7.3 gallon stainless Stout conical is $329 and a 12.5 is $399. I did alot of analysis on many different conicals when I was looking... I initially included plastic ones in my search but they were quickly eliminated for the above reasons. If they were in the $50-$100 range they are definitely an appealing option though. This is why I purchased the Onederbrew fermenter at the NHC in Seattle.