It's more than just a ground route. Syria as a state is friendly to Iran, it has not been replaced by a Pro-Saudi/Turkey government who will fulfill the wishes of NATO. Currently the SAA support & cooperate with Lebanese Hezbollah to develop assault units for the Golan. The loss of Syrian theater would create a number of large problems, firstly being the aforementioned land route, at the moment, any Iranian or Iraqi can comfortably travel into Syria, should war break out and engage Israel in the Golan. In total war scenario, one can see people as far as Afghanistan and Pakistan go to war. Truly a multi-national coalition. A possibility that has not happened before. The Syrian theater adds a new front against Israel. Beyond this even, the geopolitical ramifications of Qatar-Turkey-Europe gas pipeline would come to fruition, Neither Iran or Russia wants this to happen. This was a big blow to Nato aspirations.
I would take a different approach to what is on the surface. Try to look at Syria not at how it is now, but what it can become in 10-20 years. Syrian Military has more potential than Hezbollah can ever have just simply by the fact that their land and human resources are much more compared to the smaller nation of Lebanon. His country is in a tough spot, and so are the people, who have lost a large portion of life and suffering is immense in this country. The will for more war is not particularly high, but with the proper support he is willing to engage in Idlib, and work out a deal with the SDF. Unfortunately for him, the presence of Turkey and USA complicate things. Assad like his father is trustworthy, listens to instructions, and believed in Iran to stick by her during this difficult time. The wealthy PG nations tried to frequently bribe him with money and he still stuck by Iran, and did not accept this trap. I understand he has always been a friend to Iran, and we are repaying him for his support.
The people whose business model has become a frightening reality for many rivals, is applying this same model with Syria, I know Iran can infact re-arm Syria with force multiplier weapons beyond the current simpler systems they currently have. But it will take time to cultivate, and no amount of airstrikes can stop this process. I will say, this is a long process for Syria to re-establish its deterrent level. But it could end up stronger than it was in 2010 with this large growth in military relations between Iran-Syria-Hezbollah. More dangerous than before.