That's what people said about Delve before it came out and the opposite turned out to be true...
The only way I see this league slowing anyone down is if there is some mechanic that ties Blight mob deaths to towers (IE: mobs will only die if they are affected by a certain kind of tower and/or are immune to certain types of damage, dictating tower usage if your build uses that damage, etc.); which seems unlikely... Without that kind of mechanic, people will more than likely blaze through Blight mobs as fast as they blaze through everything else and will probably ignore the towers entirely outside of whatever endgame boss(es) that exist with the league that may actually require their use for the boss mechanics or something along those lines.