There's a couple of issues with this line of thinking.
Firstly the 3090 makes 1.5x the money the 3080 does and while it costs twice as much most miners are in it for the long haul which means that making 1.5x the crypto every month until the end of time is far more appealing than a relatively minor saving in initial outlay (plus it's easier to find which is a bonus).
As for AMD cards sucking at mining, this is quite comical to hear as for the vast majority of time mining has been the domain of AMD, hell there was a time when an Nvidia flagship could barely keep up with an AMD midrange card at mining but new algorithms/coins came out that specifically targeted Nvidia hardware and you can bet if the pendulum does ever swing too far in Nvidia's favour the same will happen again for AMD architecture.
Don't forget that at launch many considered the Vega cards to be bad for mining, however refinements in mining tools and fine tuning resulted in them being so prolific for mining that more Vega cards ended up in mining farms than in gaming PCs.
At the end of the day the 6900xt, 6800xt, 6800 and RTX3070 all perform around the same but with varying costs, however as mentioned above initial cost isn't a real concern for most miners. Most will simply pick up whatever they can find and eat the cost difference.