"How about if you chop the top and you have unworked lower trunk? same thing!"
My point was you lose at least half a growing season on the lower trunk waiting for the air layer to take. In that time, you could be cultivating a new set of lower branches, sort out the nebari, roots, etc. You can't do ANY of that while the air layer is in progress and given the couple of months (or more) that it takes to get the air layer, the season for work on the lower half has passed. All that for what? a piddly probably straight piece of stock that you will waste resources on before realizing it's not worth the trouble.
Concentrate on the piece of material that's worthwhile, NOT on simply getting another "free" tree. Free is mostly worth what you paid for it...
"My sensei said" "no tree is not worth for bonsai if you know how to approach it!"
Well, OK, we'll assume that every tree has potential...(which isn't true, or at least a very subjective opinion that leads to some very bland bonsai). The approach to an air layer of a mostly featureless, uninteresting sapling will mean about ten years of just letting it alone--all for what will most likely become a featureless uninteresting bonsai...A growing bed full of saplings is not bonsai, it is farming. You have to learn to pick your fight...