Johnson is trending the way of Payton. There's been no appreciable progress in 3+ seasons and they probably don't want to commit to him beyond (maybe) his qualifying offer. It's clear he needs a change of scenery.
Spurs were supposedly interested as recently as last season, which means Engelland must be confident he can fix his shot.
He's a long shot to amount to anything worthwhile at this point, but has a near ideal body type (6'6'' - 245 - 7'0'') and given the situation the Spurs have put themselves, is the type of flier worth considering. As a young veteran, he'd also fit with their attempt to serve two masters simultaneously: present and future.
The Pistons need shooting and backup bigs. Bertans and either a 2nd or the rights to Milutinov for Johnson and Ellenson, is worth a shot.
I'd rather not give up on Bertans, but it doesn't look like Pop is ever really going to trust him, which renders him a highly paid insurance policy.