this might actually be bad news for the Texans. SA alive meant that he could be forced to sign a peace treaty, and the rather large number of Mexican troops nearby (led by the very competent Urrea) were forced to retreat because of it. If SA is dead, then there is really nothing to keep Urrea from attacking Houston... except for some rather severe logistical problems. But if he can solve them, then Houston is going to have a real fight on his hands, facing some of the best Mexican troops led by who was likely the most competent commander (on either side) in the field...
Without Santa Anna, there is no treaty of Velasco. That's a problem, in and of itself. But given than neither Texas nor Mexico ever ratified the treaty... maybe not so much.
Urrea was probably Mexico's best general in the field, but he was under the command of Filosola, who demonstrated no interest in continuing the war with Texas.
But, what were Mexico's capabilities really like in the days following Santa Anna's defeat?
The Mexican Army of Operation (as it was called), totaled no more than 5800 men, and that at the beginning of the campaign in January 1836. By the end of San Jacinto, that number was substantially below 3,500, perhaps as little as 2,500. Although, admittedly, google hasn't been kind to my searches.
After San Jacinto, the remains of the army under Filosola and Urrea were centered around San Antonio to Refugio. This was primarily to keep supply lines open, as supplying the army from Mexico's interior was impossible. Supplies had to be brought in by sea.
But as to the issues Filosola and Urrea faced, the single biggest one is that the two men hated each other. In a continued campaign after Santa Anna's death, there's no guarantee the Mexicans under these two generals would coordinate.
The other issue is that with the destruction of Santa Anna, the Army of Operation lost some of its most seasoned soldiers. Many of the men Santa Anna had taken with him were his most experienced troops, heavily weighted toward his grenadiers and light infantry. Another thing is that many of the troops left behind were recent conscripts, poorly armed and uniformed.
Add into this that there were several ships sailing under the Texas flag that were starting to attack Mexican shipping in the coastal waters. What were they poaching? Supplies for Santa Anna's army.
Just to recap, in the aftermath of Santa Anna's death, Mexico would have 2,500 poorly supplied soldiers, under a did-unified command in and around San Antonio. I suppose it may be possible to make a silk purse from this sow's ear, but it's going to take a lot of things breaking Filosola and Urrea's way.