It was the kiss of death back then to be compared to The Beatles -- earlier victims of this phenomenon included The Raspberries and Bay City Rollers
The Bay City Rollers? - they would more comparable to the Monkees.
The Beatle comparison didn't seem to hurt Cheap Trick, at the same time the Knack were on the charts.
Then there was Big Star...who should have been big if they had been promoted better.
Utopia's Deface the Music came out in that same time frame of 79 -80.
So I don't think it was the obvious Beatles image lift that did the Knack in.
Rather, it was more likely the rather shallow and misogynistic lyrical content, plus the fact that thier second album was a carbon-copy of the first.
A lot of the "Knuke the Knack" hubris came from the rockers of the time, who all of a sudden found their New York and Los Angeles playgrounds inundated with "skinny tie" bands.
I remember reading an interview with Kevin DuBrow, where he said that The Knack and it's wannabe clones left the LA hard rockers practically jobless, from late 1979 into 1981.