Yes, by far the most likely situation is that he got distressed on the trail due to heat and took a wrong turn onto private property or dense bush in a canyon (plenty of it at Pleasanton Ridge) or a fall...and simply hasn't been found.
Let's recap a bit for people who haven't followed it closely:
1. He's from Berkeley which is MUCH cooler than Pleasanton as it's on the other side of the hills next to the bay so he wouldn't have been acclimated to 100+ degree temps and it was an extremely hot day (106F in the shade). Plus that trail is very sun-exposed, making it much hotter.
2. He ran in Pleasanton before picking up his family in Stockton, and Pleasanton is on the way.
3. Dogs traced his smell to the trailhead but lost the scent. Apparently police say this is not necessarily surprising.
4. Shoe prints matching the Saucony shoes he wears were found.
5. It was so hot that few people were out there, but apparently another runner did see him
I really wish people would stop wildly speculating that "he disappeared on purpose" and that kind of stuff. It's disrespectful to his family and friends, who are suffering, and given he's a member of the running community he probably has friends on here. That kind of stuff happens in movies, but it's extraordinarily rare and there's zero reason to think he has done this nor that he'd successfully pull it off.