I replied to this a while ago, but it occurred to me just now that no one (no one I’ve seen anyway) has mentioned the additional un-sprung weight that a larger cassette adds to the rear wheel.
1x systems ARE as good or better than 2x and 3x systems. If they weren’t the pros would not be using them. But what’s best for a pro isn’t always what’s best for a more casual rider who isn’t in pro-level shape. I am not at all embarrassed to admit that I fall into the latter category.
That said, and I assume the bike manufacturers have considered this, adding mass below the suspension (i.e. anywhere on the wheel itself) has negative effects. Will 1 or 2 more sprockets on the cassette add enough weight that it makes a significant impact? On the high-end cassettes made from expensive materials, definitely not. On the lower-end cassettes made from steel, possibly. Would it ever be enough of a difference that an amateur would notice? I think it could.
I run tubes, but people RAVE about how much lighter the bike feels and how much sharper it handles after going tubeless. Now pretend you’re comparing the weights of 2 cassettes – one with a 1×12 setup, and one a 3×7 setup. Assuming both cassettes are the same material, the one with more gears is going to weigh more. The front chainring on the 3×7 is heavier than the 1×12, but that weight has a reduced impact on handling (on a full-suspension bike) because it’s above the suspension rather than ‘below’ it. the chainring mass is no different than adding a slightly heavier rider, whereas the cassette mass is rotational weight that the suspension now has to move to work.
IF people are really being honest about feeling a difference between the few grams saved from losing their rear tube, then 5 additional rear gears would be at least as much mass, albeit not as far from the point of rotation.