The Founding Fathers believed that the separation of powers was an important principle. The Tenure of Office Act was an offense against that principle in that Congress was interfering in the rightful powers of the President. The second article of impeachment adopted in December was likewise an affront against the separation of powers.
Apparently there were seven senators who were willing to be the one to save Johnson from removal if need be. At that time 10 Southern states had no representation in the Senate. The man who would have taken over if Johnson was removed was a Senator who voted to convict Johnson.
Interestingly, this issue sort of came up again after President Trump fire James Comey.
Democrats argued that Comey had a 10 year term and couldn't be fired. President Trump said that he had the authority to fire anyone.
Constitutional scholars said that the 10-year term was a maximum term, not a minimum, and didn't protect against the early removal from office that elected officials enjoy via their constitutionally defined terms.
-PJ