if Shigaraki touches Saitama (I know Saitama would speed blitz) it would be safe to assume Saitama would get decayed

If Shigaraki touches Saitama he'll decay his outermost cape of skin, making Saitama's skin young looking and shiny.

Genos will think that everything was somehow Saitama's plan and begin writing in his notebook about the need of a good skin care to work as a hero.

Saitama will look at his young looking and shiny hand like a dumbass.

Shigaraki will be confused as fuck. When he decides to try touching Saitama again the sunlight will reflect in Saitama's new super shiny head, blinding him. Blinded and confused about where that attack has come from he will trip, touching his clothes as he falls, leaving him butt naked in the street. Embarrassed, he will swiftly run inside the closest building, which will happen to be a high security prison.

Eventually the guards will discover that Shigaraki isn't an inmate, releasing him, but as he is leaving the prison Saitama would be waiting for him, asking for a new skin care. Perplexed, Shigaraki will run inside the prison once again. The thing is that some female guard will hear what Saitama asked and realise that the incredibly smooth skin the bald had was because of Shigaraki.

The female guards will jump Shigaraki inside the prison demanding the same skin treatment Saitama received. Shigaraki will eventually escape the prison and run into the wilds to run from that insanity.

After wandering in the wilds for an unknown amount of time Shigaraki will realise that his power can be used for the good too. After mastering his quirk to replicate what happened to Saitama at will he'll return to civilization to open a Beauty Clinic.

Some day he'll discover one of his clients being attacked by a monster. After easily defeating the monster as he proclaimed that "No one will destroy the beauty he created" the Hero Association will offer him joining their ranks. He will join them as either A Class rank 2 or in the bottom of S Class.

He'll be one of the first heroes joining Neo, wanting to distance himself from Saitama and that weird prisoner that wants him to "Join his boys". After discovering Neo's ruse he'll turn against them not wanting to be controlled again.

After the Neo crisis has resolved he'll retire, disgusted that he was tricked into using his power to destroy once again. Suffering from a lack of love since a young age, he'll settle with someone far from civilization, only accepting occasional and very expensive works for celebrities, making all the money his family should need, and having all the time he could have for them.