My two cents worth!

Michael and Fredo, sentiments aside....

Fredo was never really in the family business

I reckon Fredo will always be a threat and liability, his sort of deep resentment never goes away, always simmering underneath

Roth could tell Michael was suspicious or at least had his doubts near the end but still kept at it - Tahoe shooting, Cuba, Senate hearing
I believe, a man like Roth, if he was alive would risk going through it all again until he had Michael dead

If not Roth, another of Michael's enemies would reach out to Fredo again and again, to finish the job

Even if he were exiled and cut off entirely from Michael, Connie even Tom would probably keep in touch and could inadvertently let slip and reveal something of value

Even the authorities like FBI could reach out to Fredo, to try and get any damaging stuff about Michael from him that they could use against Michael

Michael had made too many enemies Too many variables not exactly foolproof No guarantee whatsoever

Out of sight does not mean out of problems