causing a transaction backlog and fee pressure NOW kills off the desire and utility of bitcoin.
How do you know? Any evidence or just more empty conjecture?
Both transaction fees and price steadily rose from 2015 until the end of the run in December, 2017. Users were obviously willing to pay higher and higher fees. When Bitcoin has even more utility because of increasing adoption, a robust LN, sidechains, smart contract ecosystem, etc. I think that would justify higher fees yet.
Larger blocks cause propagation delays. This increases orphaning rates, which disproportionately hurts smaller miners
propagation is currently SECONDS.. so SCALING block sizes to atleast get passed the implied 600k transactions a day limit is not going to cause issues.
That's the best case scenario, and it's with a base block size of 1MB. Witness data is not a problem for the quadratic sighashing problem, but increasing the base block size is.
Some contrast to your "data":
In essence, doubling the size of a transaction can double both the number of signature operations, and the amount of data that has to be hashed for each of those signatures to be verified. This has been seen in the wild, where an individual block required 25 seconds to validate, and maliciously designed transactions could take over 3 minutes.