1999 Mazda Mazda3 - Brake Flush Necessary?

Is it really necessary to have a brake line flush?

No, but you increase the risk of brake component failure. If and when you need a new caliper or wheel cylinder, etc. have the fluid flushed and replaced along with the needed parts.

I have one done with new brakes, luckily mine last 80k miles for new brakes, so I consider it good maintenance. But then again at 197k miles I have done front and rear differential, trans many times, transfer case and ps fluid when lines busted. Course I may only keep the vehicle another 100k,

A properly done brake fluid flushing will only help, never hurt. As a practical matter I usually flush the brake fluid at the same time I’m working on the brakes for some other purpose, not as a job unto itself. If the manufacturer suggest a routine flushing be done at a certain number of miles (or at a certain time), even if no other brake work is needed, good idea to have it done. The brake fluid can get water in it over time, and the water will corrode the brake system parts and that can result in unsafe brakes and a major repair expense.

I’m in the 3-year brake flush camp. Same with power steering.
Brake pads last me typically 70K miles, which takes 10+ years (weekend driver).

At this point in the car’s life, the brake fluid will come out like tar rather than the nice clear golden color it SHOULD be. I’m guessing this because you are asking this question. Brake lines should be flushed every 3 years or when changing brake pads, whichever is SHORTER.