Any coin you'll buy from an exchange will probably have a segwit history.
Which of course would just result in a niche that needs filling by an entrepreneur to found an exchange that deals strictly in Segwit-free bitcoins. Market pressures always result in market solutions. Kinda econ 101, that.
There are no such exchanges, and I seriously doubt any future exchanges will exist that deals strictly in Segwit-free bitcoins.
There is NO difference between the "distinctive classes" you have enumerated. None. There may be a technical difference, there is no practical difference. It's like saying cracking 256 bit hashes or encryption has a chance. Yes, there is a miniscule technical chance. No, there is no chance for the normal person or government, or governments. The chance of winning the lottery is higher.
I have continually stated that I am merely pointing out that a reduction in fungibility is endemic to the design of Segwit. I said nothing about a current attack.
There is no attack. There won't be an attack. Not while we live (for the next 50 years). If coins have no difference other than what address they came from, then they are fungible for all practical purposes. If coins are mixed through many different ways (going through an exchange, Coin Joined with hundreds of others, coming out of some gambling site, bouncing from one address to another) there is no difference among all of them, they are all valued the same.
The worth of legacy coins is the same as the worth of segwit tainted coins. It's all bitcoins. You send them to an exchange they will pay you the same rate as any other sender.
The difference is, it's a little bit cheaper to send segwit coins. It costs the same to receive them.