Try sending ACH to your landlord or your Gardner or DMV. Or that class action settlement Check you received - why didn't they just ACH it to you ? In the former case - entities don't want to give out financial info to people they don't know. Even if they do how do you plan to initiate an ACH to your Landlord ? In the latter i.e. a business or an individual trying to pay you must go through an extensive process to collect your bank account info, that's assuming you are willing to part with it.Donald Knuth doesn't write Checks for this reason: http://www-cs-faculty.stanford.edu/~uno/news08.html
About ACH not being ubiquitous. Try ACH'ing from/to your Money Market account. There are over 100 million of them in the US. It won't work. Small Banks and Credit Unions - yes appx 12% of Credit Unions are not ACH enabled (stats are behind the NCUA firewall ). Just because large banks in large metros use ACH ubiquitously does not imply universality. But regardless that's just one point for one demographic. The big picture is getting payments without sharing bank info.
About your point about CapitalOne and Comcast not processing a Check as Check - that has been around for decades. Truncating a paper Check to ACH transaction to get rid of paper. We are going one step further. Not generating paper Check to begin with. If there is no paper Check there is no need to convert (i.e. truncate in bankers parlance) to ACH i.e. saves another step in the process and makes it more efficient
This is not about non-ACH banks, its about getting rid of 3 day ACH delay and building an experience where anyone can send a payment to anyone else without signing up, without going thru the cumbersome bank UI/UX, without sharing their bank account info and have it delivered in their mailbox. Free for individuals (up to 25 Checks) and businesses pay $1/Check.