You can already do the Gold Saucer at a low level (from level 15 when you can travel to Ul'dah). New content and rewards are added to the Gold Saucer quite often, in each expansion.

Wipes do not happen of proper MMOs like FF14 with over a million active players. There is another MMO called FF11 produced by this company that has not been wiped in its whole 19 years.

Given FF11 has not ended after 19 years, we can expect FF14 won't for just as long at least unless Square Enix went bankrupt, which is not likely for a company that produces dozens of worldwide games a year.

The "adons" are called patches and major patches release every 4 months, with smaller patches every 1-2 months.

You may find this useful https://na.finalfantasyxiv.com/lodestone/playguide/