The sniper rifle, when used properly, is indeed a devasating weapon with "one shot, one kill" capability for anything remotely human.
The reason to use/research more tech is to have other options. Sniper rifles are not the most useful weapons for clearing rooms, for example. I suppose they can be adopted to that, if one moves very, very slowly. For example, in a highly urban setting my sniper on the roof can often see an enemy, but not hit it. When moving cover-to-cover, a sniper may not have enough time to find the target, shoot, and cover. In other words, sniper is a designated killer who needs support, at the very least in shape of scouts who id the targets.
All that being said, maybe I will stry a game with a team of 8 snipers only, just to see if it can be done. Off the bet - capturing anything alive would be very tough with a team like that.