Well, there is a lot of thought and preparation put into making a roster that can successfully play a blank lineup. In that respect, it's no different than a team who sets actual positions.
You're making the assumption that teams who play blank lineup, play it every single game. I don't think that's true. People do scout opponents to decide if a blank is worthwhile, and then balance it with other needs of their team.
You seem to assume that people who use blank lineup never set any positions. That's not true. Many teams may blank only some positions, after making the exact same tactical decisions a non-blanking team does.
You're making the assumption that teams who set their positions, actually switch things around a lot. I don't think that's true. Many teams run the same players out in the same positions no matter what.
You seem to assume that there's no downside to using blank lineups. Well, managing minutes can be somewhat more variable, and getting consistent minutes in certain positions (for training) is more variable. So managers are making a strategic tradeoff when selecting this option.
I wouldn't really care so much, except for the fact that many of the substitution patterns don't seem to work correctly. If choose from depth chart or DC until 4th worked well, then maybe people wouldn't turn to using blanks.
The issue about LI/M2M is being addressed through changes in the GE and in salary/cap costs.
Lastly, if you're just setting LI/M2M and blank, then that's your problem. If you don't like it, don't do it. If you're bored with LI, construct a different team (JBBL seems like a place where you could get away with testing stuff. Really, that doesn't seem so hard to understand. If you want to mix things up, use different strategies, nothing is stopping you.