The didn't have the healthy pass rushers on the line to run a 4 down front with Hunter hurt and Griffen ill.
The definitely don't have the pass rushers at the LB position to run a 3-4. Especially with Barr all but assuredly not being on the roster next year.
I don't care which way they go, but they need to find pass rushers either way.
Nobody in the NFL runs a 3-3-5 base defense. Its not even a thing. Works in college, but as it can be torn to shreds by a super accurate QB as we witnessed when that Texas Tech QB was dropping dimes. Problem is, most NFL quarterbacks are dropping dimes. Plus, you need good pass rushers either on the DL or LB area anyway.
Not really sure how you see an alignment change as being the solution to the Vikings pass rush.