I think you don't understand because you're an enthusiast and they're transportation appliances. I bought one of the early ones new, 2002, which had among the more dubious reliability records. I bought an SL, which had all the luxury stuff and the 2.5L. I was attracted to it for several reasons. While the 4 banger lacked grunt, it was as you said "adequate" for my non-enthusiast wife & son. It was also significantly larger than it's 2002 competition at the time (Accord/Camry) , with a big trunk, comfortable roomy interior, all the luxury goodies, was priced very cheaply, and it got well over 30mpg on the highway. It was a very good family hauling transportation appliance, which is why it got so popular. The fact it was not a canyon carver didn't bother me as I had other more fun vehicles.
Reliability was very good for me for the first 7 years but ultimately Nissan's refusal to cover the widespread catalytic converter disintegration/oil consumption problem under warranty meant a premature death to the engine (about 175K miles). That oil consumption was the main reason I (and many other owners) got rid of it. But since my kid got 2 yrs out of it before it got real bad, I was little less annoyed. But it reduced my confidence in buying another new Nissan.
Fast forward a few years and Nissan switched the Alti sedan to an awful CVT, eliminated the man pedal, eliminated the SE-R, and the only real investment they put into it was the gimmicky memory foam...er wait...I mean "NASA inspired zero gravity" seats.

Like the rest of the Nissan/infiniti lineup, they haven't done much development, which means they've fallen well behind the rest of the industry who have chosen not to be so stagnant with their sedans. No surprise that it's now a low ranked car. I suspect appliance buyers might finally begun noticing Nissan's laziness.
As far as the coupe goes. I'm sure younger non-enthusiast folks have been liking them because they're cheap to buy, cheap to operate, cheap to insure (not sports cars), handle okay, roomy, and look good. A winning combo when it first came out with the same downside now: Nissan went cheap on development and killed the fun by making it CVT only.