If you're uncertain, I'd hang onto it, a 90mm 2.8 Elmarit can't be that much "dead weight"? I too hope 120 film stays in production, but if it doesn't, I wouldn't go digital. After 10 years of enforced digital shooting, due to media demands; I was glad to exit the media world and get back to film. Digital is a financial "black hole" with "new improved" models quickly making your current model obsolete and plunging in value. (an "X" or an "S" added to the name does not constitute any practical improvement in my experience).
I was recently asked to supply a digital image I took back in 2003.....loading up the disc, I found most of the images seem to have "gone soft" and some had corrupted altogether. About the same time I'd scanned some black & white film negs. taken in the 1970's.....they were as sharp & contrasty as the day they were taken. I'd been told stored digital images on CD's were unstable, but hadn't personally experienced it till now.
To get back to your main point, I once sold a 105mm Nikkor lens that wasn't being used much at the time.....I regretted it after, and have since purchased another S/H. It's now used regularly with my film Nikons.......If it's a quality lens, then hang onto it.