The US does not use imperial units. Imperial units are those defined in the British Weights and Measures Act of 1824. What the US uses is the US customary system, where units have the same name as the Imperial ones, but not the same values. Now, the US customary units are defined in terms of SI units anyway, just with annoying, non-integer conversion factors.
EVEN BETTER
(where "better" is actually "really annoying")
Actually, no. The US gallon (3.785L) is not the same as the imperial gallon (4.546L), a point which made me see red when Canadian opponents to metrication argued that we must keep the gallon because the US is our biggest trading partner.
The real reason Canadians and Brits should have opposed metrification: the transition wasn't ever 100% completed leading to horrifying Frankensystems where Brits buy meat by the kilo but measure their own weight in stone. Or Canadians still measuring train rail distances in miles.
Then just push for full adoption of the system and over time youll get there. Can be slow and sometimes painful but so are a lot of things; "no pain no gain" as they say
You have it backwards. Metrication in Canada stopped halfway because the opponents created such a stink that the government of the time decided to back off, and nobody since has had the courage to seize the nettle. Perhaps in a generation today's schoolkids, educated in metric, will try again.
To rephrase te question: why would somebody be annoyed with the fractional metric size of A-n sizes ?
The answer is probably quite subjective. In my case, when I deal with materials such as wood, cardboard or paper I enjoy immensely to have dimensions that snap to a certain grid. Call it a mental defect if you will. To illustrate further, let me propose a strawman: would you have no preference between the A4 size being: (1) 210.32 x 297.912 or (2) 210x297? Does it not bug you that 210.25 was rounded down to 210?