What makes you think "is light aerial construction the limiting factor"?

The structure has to designed to reliably and safely support the weight of the vehicle, operating systems, and people... As has been noted that would include limited systems factors like platform length and power and guideway requirements. Add to that the propulsion system choices like on vehicle motors versus a linear induction guideway to propel the vehicle, creating additional loadings which the structure must also support, as well.

What makes you think that structural need is somehow this need is less when it's a monorail? If the vehicles full of people "in total" actually weigh less, then "maybe" yes, depending on other factors (soil and foundation, earthquakes, etc.), if not, then no....

This sounds like one of the more ludicrous monorail (but more often Maglev) myths, that they somehow require less robust structures. I once even heard from a high six figure "executive director", who actually had come to believe that since Maglev "levitated" on the tracks that the structures would be so much cheaper, because since they "levitate", we had somehow repealed the law of gravity, these could somehow be "lighter"...