Why? Devs can lend the troops for each battle (level 20, full traits) 
.
Moreover, they would solve the issue of “dungeons too difficult for low level players”.
And these players could use these troops in Dungeon battle and so on, have some hype to get them (yeah no option to buy the troops like in Arena).
We can even imagine that it’s the players who choose the invade teams (or one team by week) similarly to what it’s proposed there.