extra weight = a few hundred grams, not exactly loads.
A few hundred grams for a relatively cheap change is a lot!
extra cost? – eh? – most people seem to need new chainrings, new rear mechs, etc. (and while they’re at it, why not ‘upgrade’ to 10speed)
No its cheaper, one less shifter, and no front mech, if you are upgrading anyway.
no real benefit? – granny gears can be very handy.
Can they? I suggest you get fitter. For some riders granny gear may be useful, but, in theory, as you ride more you get fitter and you can use bigger gears, not that the difference between the bottom end of a 1×11 and a 3×10 is actually that great.
Just as a reference the lowest gear shipped you could get at one time was 26×30. and plenty of cheaper mtbs came with stupid ratios like 30/40/50 13×30!!!!!