I thought the problem was you couldn't cap with anything other than something with minesweepers or else you risked losing the squad. Which is a pretty awful problem.
This is a huge oversimplification of the matter. First of all, capturing points with a lonely squad always means that you risk it, apart from early game where there are no demos either. You might get flanked by infantry blob or AI tank. You might get gunned down or granaded by an infiltration squad such as commandos or german stormtroops. You might even stand upon a goliath or booby trap. So no, old demo is not an outlier here. It actually blends in perfectly.
Another thing is that it is not exactly true that to avoid being demoed you need to always take a sweeper squad with you. Many players used to estimate the position of enemy combat engineer squad and send sweepers only to the points where they used to be. It is a very effective way of narrowing down the places where demo can possibly be. Players also used to play mind games similar to the one caused by mines - guessing which spots are popular for demos and trying to move your capturing squad into a position where a demo is very unlikely, also choosing accordingly safe path by for example jumping a fence.
If demos were only possible to set up on building and cover, it would be much easier than that to avoid. All you would have to do is send your capturing squad in the open ground and you would be pretty much safe.