Isn't that always in the serious fan's purview?
I agree with all of those points. Your point 3. is often overlooked: potential replacements for the following year's FAs. Your point 1. is a little controversial but I think there's truth to it. Bahktiari was a surprise himself, first earning the starting job week #1 while showing himself to be a good pass blocker right from the start. With Spriggs they may have been looking for a repeat with huge cap savings if he pulled it off. Unfortunately, it wasn't even close. Plan B: Bulaga's a FA after 2019; he'll be 31 years old in 2020. While that's not terribly old for a OT, there's always a concern given his serial knee injuries over the years. He may not have any MCL left in one of those knees. Spriggs will also be a FA after 2019. He's going to have to show something before then. His problem in showing no development now may be discouragement in the fact he sees not chance to start prior to 2020. That, of course, would be a big mistake.
We might add Josh Jones to the list as a 2-factor consideration.
Factor 1: They were using Burnett last season as an ILB hybrid and Thomas as the dime backer with Martinez seeing fewer snaps toward the end of last season. We have to conclude they are not happy with the Ryan-Martinez combo for anything other than base/run downs. Even still, as noted before, their chemistry in the run game still does not look good as they run into each other in competing to get to the ball carrier first. I think Martinez is the loser in that equation.
Factor 2: Burnett is a free agent. Jones is insurance if Burnett starts showing age. I would not assume they will not re-sign Burnett. If he plays this year to last year's level I'm sure they'd love to have him back at age 29 on a shorter term deal. The market for 29 year old strong safeties is not super strong unless you're an All Pro. But Jones helps cover the bases. And for my money Burnett had a Pro Bowl caliber season last year and if he follows that up somebody with an acute need could price him out.
Everybody wants more than one reason when making a significant decision, if they can find them.