Before ~2010, if a UK citizen married a non-UK citizen and wanted to get them residency and ultimately citizenship, it would be relatively simple and not terribly expensive. It took (IIRC) ~1 year to get ILR (Indefinite Leave to Remain; basically a green card) for their spouse and 4-5 years to get citizenship. In the US that takes 4-5 years (1-2 years to get the green card then 3 years to get citizenship) generally.
Now that process typically takes ~10 years minimum, has all sorts of requirements and will cost you in excess of 10,000 pounds once you add up all the fees along the way. The requirements are things like a "Life in the UK" test but now also a hefty income requirement, going up to (IIRC) ~38,000 pounds per year from next year.
So I have to look at all moves the UK makes in this area through this lens of increasing isolationism post-Brexit. Physical residence cards do have advantages. You can use them as ID. They clearly have disadvantages too. For example, US LPRs are technically required to carry their green card on them at all times. If they lose it, a replacement costs ~$500 (last time I checked) and takes roughly ONE YEAR to replace. If you lose it while overseas, well that's going to be a huge problem. US citizens can get a passport within days if they try and within a day or two if lost while overseas from a US consulate or embassy.
At the same time, there's really no need for a physical card. Any government official or financial institution or employer should be able to verify your status electronically. I can't help but think that this is somehow designed to make life more difficult for legal migrants in some way though.