The honest, straightforward monthly fee is a big part of what got me on board with trying DU in the first place. ~$7 is very cheap by MMO subscription standards, if that does not cover the server costs of an active player it should be increased. Especially with no base game cost, it's very affordable to try a month.
A flat game charge of even $60 is woefully inadequate for a true MMO with large demands on the server created by all the building possibilities in the long term. Space Engineers can do it because it's not an MMO and they don't host the server. With most non-mmo sandbox building games like these, your friend group has to chip in to rent a server or one guy pays it for the rest, but regardless someone is making monthly payments (even if they host it themselves, they are using the internet and electricity they pay for). Recurring costs are a fact of life for online games like this.
I could tolerate a cash shop of purely cosmetic items for our avatars and such, but the Free-to-play model is corrosive to the soul of games. Gameplay design decisions inevitably are affected by efforts to drive players to spend more. If DU goes to such a free-to-play model, I am out.