Why do you think tsunade never learned sage mode

[Ansatsuken]

That's exactly my point. See what happened when he did? Everyone cried BS. Same with Madara/Obito/Danzo playing plug and play with Sharingan eyes.

Could you imagine the outcry if Tsunade busted out with Sage Mode as well?

Now don't get me wrong. I think she has the requirements to learn Sage Mode no doubt. But the damage was done when all these other sages started popping up during the war:

Hashirama
Kabuto
Minato
Then Madara

Sage Mode is not the 'rare category' material where Rinnegan is one and its Okey. It never been intended to be 'rare' unlike Rinnegan.

There will be no Outcry if she managed to attain it at least if that SM is base on her domain, Healing art of Slug - Slug SM. Many people hope it to happen.

SM is vary between each others where we can see its uniqueness. A degree of difficulty to achieve it doesnt make it rare but making it less cheap. SM Minato was a fan service but that wont diminished SM exclusiveness

Ordinary Rinnegan beside the same standard tech it granted to the users(less unique), the requirement to awaken it is not easily accessible. Its 'rare bcus of that(The requirement) alone and if more people managed to awaken this eyes, it will be too cheap and lost its rareness. And it should be that.

 

[InfiniteDeath]

Her instant regeneration technique is probably the equivalent of a slug sage mode

 

[iFlowMotion]

She definitely has the Chakra levels lol XD

Short answer: she honestly doesn't need senjutsu. She already has impressive speed, stamina and overboard strength in addition to regeneration so SM would be an overkill honestly but she's well beyond capable of learning.

This ^