If the FBI wanted to investigate just the four people, they could have done it with regualar warrants. These people were just used to get a FISA warrant with its two-op surveillance permission. Nearly every person in the Trump circle, up to and including Trump, could be caught up in it. They were probably already being spied on, but the evildoers in the FBI, CIA etc. needed something to make it legal.
Just that would have been illegal on the part of the CIA, NSA — and possibly the FBI if they had not warrants for surveillance. But you’re right — we shouldn’t be naive enough to think that because it’s illegal that it doesn’t go on in our government. However, if they are going to take it into a court of law they need to be able to say that they obtained this information lawfully — and that’s where FISA comes in.
If the Trump-impeachment results in the repeal of the Patriot Act & cause a whole-sale restructuring of the mission of the FBI it will probably have been worth it. It may take a constitutional-crisis to end bad laws & rogue agencies.