I've run games for multiple groups. Even with the same options, there's a big difference based on the group. So no, there can't be one blanket rule that covers all groups.
Consider some of the variations:
- magic items
- feats and multi-classing
- mix of classes, party tactics
- number of PCs - larger groups are much more effective
- DM/monster tactics such as having dragons flying around using hit-an-run tactics or landing and slugging it out in melee
- Lair actions and Legendary actions
- Number of encounters between rests (is this 1 encounter and done or the 8th encounter since a long rest?)
D&D is not a board game, there are too many options for the CR calculation to fit your personal table. All I can relate is my own personal experience. I've challenged groups of all levels. It hasn't always been with the encounters that I thought would be a challenge, but party tactics and luck play a big part. Yes, that included singleton monsters at higher levels, although it did require me to think about the environment and tactics that made sense for the monster.
So saying "it can't be done" is simply untrue. I've done it. From what I've seen from other posters so have many others. If a DM can't do it, I don't blame the rules, I think the DM should take responsibility for how they run their game.