Goff is less mobile....somehow he was able to be quite successful. You have to make a plan or adjust....if you can.
Different problems require different solutions. The Rams offense has Higbee, who can function as a great outlet and an efficient target when getting the ball out quickly (as TEs often are), which allows the Rams to run a quick passing attack predicated on fast passes. They also have decent WRs to allow for a deep passing game. However, being built in this way means that they are better served to win against teams with a solid pass rush but weak secondary.
The Cowboys are not built this way. They are built around their WRs and have TERRIBLE TE play and not really great play out of the slot. The offense is built around letting its talented WRs run routes and then hitting throws to those WRs. As such if the OLine is getting beat, meaning that there is no time for these routes to develop, then the offense falls apart. With the litany of injuries the Cowboys OLine has sustained, that has become the case. Now the strength of the Cowboys style is that if the OLine wins you can do well even against great secondaries, but if they lose it looks really bad. Having a less mobile QB (and less accurate and more INT prone but those are not important right now) actually makes this situation even worse and puts even more pressure on the OLine to hold up which they clearly could not do.
Now part of the issue is that they had to go against AD who was throwing 2 blockers aside at once, but right now they need ways to mask the deficiencies in the OLine so that the offense can function.