Oh noes! The DM won't interpret the spell in a way that it rapes everything for little cost! What have we come to? Oponents who make their checks geting some time off? Heresy! Casters must dominate everything, even if the RAW is questionable!
I just find it hilarious that caster players just love to twist RAW to their benefit at every oportunity, but when said strategy is turned against them, then they start complaining like this.
Seriously, I support your DM. The RAW is indeed questionable, and super grappling machine may indeed be a bit too much for a 4th level spell.
The spell it's still far from useless, as oponents still need to get out from the grapple, and it discourages oponents from entering the area, plus slowing them down, wich blocks charges and, well, slows oponents down. It is still good battlefield control.
And if you want to stay inside it so it provides you cover and makes enemies thing twice before closing for melee, good for you then!