Except Sandbags don't evaporate, they stay around until they're destroyed. If you aren't able to build as many sandbags in the early game, then you will have fewer in the mid-lategame, especially as after the early game you are going to be actively fighting over positions, leaving your units less time to be away from the action building sandbags.
Infantry does not suddenly lose ability to build them and unless you're playing top level 1v1(which clearly non of us does), you'll have more then enough time to build and fortify all you need.
If you have so little downtime to not be able to build sandbags, explain to me, how do UKF sim cities are being made? Are they paradropped? There clearly is 30-60 seconds to build them, you'll find these 20 for a sandbag.
You're making excuses about a change that changed EXCLSUSIVELY early game, because you CAN and you WILL build sandbags when you're decapping and recapturing opponents point/taking back yours, so again, stop making up excuses.