Depends on what you use your retainers for. The fishing retainer is the only one that can fetch the Fat Cat minion (as stated above).

It is more than capable of bringing the same items your miner retainer fetches with each Quick Exploration.

Frankly, it's useful enough but not essential unless you are cooking some of the CP/GP enhancing recipes. Then again, I don't send out my miners or botanists to fetch crystals (at level 50, 6 years ago, sure ...) All of my retainers do Quick Exploration these days, since I have no need for wood or stone or fish.

If you really need another miner (or, perhaps, a hunter), think about investing in the level 60 retainer jump potion if you're going to switch. And get ready to buy all that old gear. Again.

At level 70, there are no further 'fishing guild' quests. There are a few quests at the Crystarium though, which you can use to quickly advance a level or two for your fisher. If you are level 73, and your retainer is level 71, just realize that it takes a whole lot longer for your retainer to gain a level (unless you have it doing Quick Explorations) than it does for you to do so.