Effectiveness of DHT blockers

My question is; does finasteride and dutasteride lost effectiveness over-time in the body? Like, say in decades, would my body become used to dutasteride and it will lose its effectiveness and hair loss will resume or can I preserve my hair forever (until I’m a senior, theoretically) just from taking 0.5 mg of dut everyday for the rest of my life?

Another question, will the hairs that grew back become thick like how they were before they fell out? Is there anything I can do to make them thicker or do I just have to keep taking the drug and wait longer?

It is not the blood levels of these drugs that are important, but the tissue fixation that occurs after taking the drug. The tissue fixation of finasteride is about 7-9 days while the tissue fixation of dutasteride is about 6 weeks. These drugs generally do not lose effectiveness but the balding pattern is tied to apoptosis (cell death) which these hairs undergo over time. These drugs slow the process down but will not prevent the outcome, which is inevitable.

In the early stages of use, when miniaturization is high and newly formed, the probability that these drugs will reverse the miniaturization process, making these hair stronger and less miniaturized, will be good.