For Bitcoin to get that high, the assumption is that adoption will be far higher than it is now, and with that, more demand for block-space.

However, most folks who buy Bitcoin already aren't transacting much on chain. They usually keep their coins with custodians. I think that trend will, unfortunately, continue and grow. Fees will be higher in dollar terms, but fee rates in sats/vbyte will always have high and low demand periods where we will still see single digit fee rates at times.