Here’s a great argument:
It used to be like that and everyone hated it, so it changed.
Good point:
I also have a great argument:
Nobody outside uses them anymore. Even at my skill level players are good enough to avoid them in all modes. They are a waste of mun. The only times you can actually use them is hiding them behind houses, which works only on certain maps and even then about once per game.
Now go on and explain why this is exceptionally good game design.
The comparison to Tellers is obviously not 100% complete, but all in all it works. At the moment you have Somewhat comparable investment with a likely unit wipe. Just that the Teller is slightly cheaper, the wiped unit more expensive and do not require vision or manual input. On the other hand, they effective radius is smaller.
Your B4 comparison is quite off. Nobody knows where the B4 will hit, not even the person building it. And there is absolutely no counter play to the explosion. The demo charge can (at the moment) be swept by any squad without getting any damage. And in the previous implementation they could still be nullified by having sweepers around. If these two are the same to you then you are 1 billion percent clueless. This is not debateable.
But now on the more serious side again: Also the comparison to normal mines is a bit lacking. Normal mines don't give you much value if you do not support them. The fact that OST needs to somewhat rely on Tellers to counter is a tricky argument. Yes, they lack a light TD, but in many (even high level) games they survive the LV phase even without any Teller. The current implementation of Teller mines just shows that the faction design of OST is a bit off compared to others. An ability that can just turn the game around 180° is bad design, nothing more and nothing less.
I'd argue that if every squad should be able to detect demo charges, than the detection radius should be lowered. They should detect it approximately at the point were they get the outer edges of the AoE damage. So no wipe if a player is careful and pulls back, but they should get damaged.