Lowcountry Low Roller
That’s a great way to handle an “advanced” mode and actually the table approach like in Xanathar’s would be a great way to present it.And the current CRs actually work for some people. I think it is just easier for them to publish a separate encounter guideline for hardcore or advanced gamers. That with some more advanced options of existing monsters (which they are already doing) and the issue is solved., IMO.
The whole reason we have these encounter guidelines is to identify the challenge range where the outcome is uncertain. Too easy for the PCs - outcome is certain, too hard for the PCs outcome also certain: TPK! Our problem with 5e has been that the outcome of too many combats has been certain: the PCs win with ease and thus the dissatisfaction. Simply providing an alternate table where the CRs are recalculated would be ideal. And then companions to published adventures could be made available on DMs Guild to make scaling them up easy.