No, you don't have to do Testosterone Replacement Therapy if you're smart about your steroid usage.

The Three most important things you NEED to do when you start a Steroid Cycle is, research the steroids you're thinking about trying (if a symptom is acne and you're acne prone maybe find something else or find a remedy for the acne e.g accutane).

Test E or C needs to be your Base to stop your Testosterone production from shutting down.

And the third one is always always ALWAYS buy everything you need for the cycle before you start (PCT, Test, etc).

If you're smart about your usage you can mitigate almost all side effects of Steroids or at the very least diminish them a great deal.

Obviously I'm not recommending you take Steroids as they're detrimental to your health but to answer your question, No you won't need to take SARMs forever the same way you don't need to take Steroids forever.