I was thinking about this the other day. It's a challenge to keep a franchise strong year after year, with 1) shifting expectations from fans, 2) IPs changing hands, and 3) studio talent leaving. 

What franchises have stayed strong throughout the years, and why? What long-running franchises have never turned in a bad game?

Dragon Quest. Because they know what people want and they will never touch the winning formula, because if they do, people will riot.
I can say that because I recently played DQ1-6 and currently DQ8 again. It's basically the same great game over and over with a few nice tweaks to surprise the players without overwhelming them or breaking the formula.

Mario Kart

Tekken, a glitch with Tekken 4 where they added walls (but still a great game) until 5 where they got it near perfect and Tag 2 was a bouncing juggling mess but otherwise, left punch, right punch, left kick, right kick plus movement in a 3d environment for the last 2 decades have produced a formula that works amazingly and they have adapted it well for the industry.

But then Harada has pretty much over seen it since #1.

I'd say WipeOut as well but don't think we'll see another entry.

Probably the Souls series, which have consistently maintained an amazing quality while being fresh in all entries.

I'm tempted to say Metal Gear as well, being like one of the oldest 3D franchises (that didn't even debut as 3D) that also managed to release quality after quality in all its entries, but The Phantom Pain might have been a problem with the record being "flawless". Although critics loved it.

While some might disagree with me, I would say Final Fantasy.Sure that it has its ups and downs, but they all are at least great games.

And all visual novels made by Spike Chunsoft.Every single one is excellent.

Yeah :(

It's a crime that Liverpool closed down, especially after 2048 - which I thought was one of its best.

Define "flawless".

Look it up in the dictionary. It's right next to a picture of the Dragon Quest series.

Without defect. So a franchise that has turned out only good/great games. Never a poor/bad game.