Before Better Call Saul even began, Mike Ehrmantraut was considered a fan favorite by Breaking Bad viewers, but the prequel series would eventually change for the better. Beforehand, he was a new take on the action hero trope. The modern action hero archetype is young, buff, and, more than anything, invincible. That last adjective describes Mike well, but because of his old age, Mike was initially underestimated by viewers and his enemies. However, his ability to thwart his adversaries through his sharpshooting and criminal smarts always helped him prevail. Because of that, Better Call Saul didn't have to do anything to make him more compelling–but it did.

https://screenrant.com/better-call-saul-breaking-bad-mike-ehrmantraut-improve/