Perhaps the coaches don't have faith in the 2nd strong to execute without making mental or physical mistakes? Risking a fumble or forgetting who has outside contain could cost them the game. Keeping in the players that were executing without errors but physically gassed might have been less risk.
Obviously the coach has his reason(s). Your guess is a good one but I find it hard to believe that no one on the team is close enough to the starter that a snap or two to get the starter a breather would collapse the entire team.
Using my son as an example, as I know one of the RB's very well and his abilities. Really, if you matched them up there is very little overall skill difference and it is more about what do you- a small, quick RB that is a little faster or a bigger one cut power runner. Or to replace the other RB who is same type of RB but bigger and slower. There is a lot of practice I don't see even though up to last week we were on the same field practicing but the couple of times I caught my son get a snap, he had long runs (really didn't see long runs from others), at the jamboree- the only time we effectively ran was with my son (caveat on that being that that was 2nd string against 2nd string) but the most important was twice I caught my son explaining the play or moving someone into the right place. I use my son as the example since obviously my attention is mostly on him so my observations are largely based around him.
Also, the starters are all 8th graders with one exception other than a couple of 7th graders on ST.
It is what it is. But man... we got lucky last night. If their kicker didn't shank the extra point and then later the FG which he had plenty of leg for.... it would have been a loss. If we they made that extra point and we went to OT, it would have been a loss. The more the game went on the more that they were dominating.
I just think you really got to find a way to get some subs in there. If you have almos the same 11 on the field the whole game like that, we are going to lose some winnable games for sure. I mean, on our munchkin team where we have 18 players total and played with 16 in our first game (one boy is still out with a broken arm he suffered before football and the other was sick) with some big difference in ability/skill/understanding and we still got most of the boys a breather.