On the contrary, where those with PRM need essential tavel, it's most inequitable that they should be further restricted than everyone else. Inaccessible stock needs more assistance boarding, and in the journey, which can't be given easily with distancing - so if anything, to meet the needs of Social Distancing, and equality, acceleration of new stock introductoins and minimising use of inaccessible stock, would be better. Generally, new stock is likely easier to clean (by design) and should require less staff intervention in operation (as a design requirement). Well, we can hope...
Yes, longer trains would allow better distancing, but as I understand it most of the new stock already offers more space per unit (certainly on TPE and LNE), so is an ideal choice all round. Given shortage of staff (due to staff who must shield and can't work safely), running more services is not an option, so choosing the most helpful stock for the limited possible service is the best option. And that includes optomising PRM access, even if only as a side effect of best stock choices.