If Spruill was a proven MLB then why didn't he play last year at MLB? Staff wasn't comfortable with him and his assignments there. Also, Arciniega might be a stud but hasn't proven a thing yet. Cam Lynch is a good player but has been better at OLB as well. Not to mention Shafer and his assistants have always been mainly 4-3 guys.
You also need to look at the depth on the DL compared to LB
DE: Welsh, Robinson, Simmons, Manley, Trejo, I. Johnson
DT: Bromley, Z. Jones, W. Williams, Raymon, Sloan
That stable of DT's would be much better suited to 4-3, especially when you can play Raymon as a strong side DE. Robinson and Welsh played well and Simmons looks ready to make the jump this year.
Contrast that to our LBs, who are
OLB: D. Davis, Lynch, Spruill, Vigille, J. Washington, Kirkland
MLB: Arciniega, Coker, Hodge
There is no combination of 4 of those guys that wouldn't leave us exposed in coverage/against the run. Many are situational IMO