I know people for whom a ramp is as bad as stairs.
I am one of those. For anyone with impaired heart and/or lung function it is the effort needed to gain height that matters.
When I've seen news of lifts built at stations, there is often either a proud announcement of how easy this is for wheelchair users, or an admission that the site couldn't fit the size of lift they aim for. Compare Tooting and Leatherhead
I have already used the new lifts at Leatherhead, and I'm very glad that I no longer have to climb the stairs to get out of the subway.
Cost? Even TOCs would love 100% redundancy, but the cost would be double (I doubt there would be economies of scale)
There would be some economies of scale, for example in design and power supply. But, in most cases the cost of installing more than one lift is going to be prohibitive. I live in an apartment block and I'm reliant on a single lift: what annoys me is not the occasional breakdown, but the breakdowns that are not fixed the same day. For the railway, with lots of lifts of the same design, this should not be a problem as they should have suppliers with replacement components in stock.
But there should have been more than one lift at particularly busy stations, or if lifts are going a long way underground. For example, there is not enough lift capacity to/from the Elizabeth line at Farringdon, or at the St Pancras Thameslink platforms.