Ich is tricky like that. There's lots of people who reach stability with it in their system, rarely seeing so much as a single cyst for months or years. Fish can develop a partial immunity to it over time, and there's a bunch of different strains, some probably less virulent than others. Even if you don't treat these fish and fallow the tank for months, doing the TT on all future arrivals limits the potential to introduce a new strain to which your fish have less immunity and is more virulent.
You might have a hard time adding certain fish to this tank, and if you do add an "ich magnet" it will be possible for the levels of parasite to climb high enough to affect even your otherwise healthy fish, but it's entirely possible to manage a tank with ich. Lots and lots of people do it.
I went through the headache of fallowing my tank because I couldn't add new fish. New arrivals would be overwhelmed and die within a week, while my resident fish were pretty much ok. I lost several otherwise healthy fish to the treatment, then lost all of them when the QT tank crashed a few days before they were supposed to go back in the display. The treatment ended up being way worse than the disease. Since then I've worked hard to keep it out so I never have to deal with it again.
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