I'm too wobbly to analyse the reasons as to why the SimCity series died, but I'm sure it involves all of the above criteria.

or pretty much all of them together, like what happened to Sonic

It did when?

It only died for me because it's an EA game.

Deviation does it for me... I don't mind the others that much unless they are really extreme.

whats the one called where they get a new box art, make small changes, add in dlc and sell it as a package to piss off the people who paid full price for it?

games include:

super street fighter (ok this one had reasonable changes but still...it makes you buy the game twice)

resident evil gold/move edition (sure they didnt have enough memory for a move patch)

GTA "complete edition" (feels very nice to own an incomplete version for which i paid 4x the price)

all "GOTY" editions which were not even GOTY (note how its not game's' of the year, its game of the year so how can borderlands have it when uncharted won 100 GOTY awards?)

batman arkham asylum GOTY (why didnt they do the 3d in the first place?)

burnout paradise the 'ultimate' box

What's wrong with tiny upgrades, if they're there? Provided the game is any good to begin with, that's the way to go.

Also, as long as the original game series isn't abandoned, an additional well thought out spin off is ok. If you'd always reject that, you couldn't play Mario Kart or Paper Mario.

If the content in the Gold edition consists of what is already for sale, why not release it with the DLC?  People who have the original still had the chance to get it.  What I was considering is the case where the sequel is even less than a Gold version, with additional content involved.

If you can roll it out as DLC, why not go that route?  The issue is that, if it is an expansion pack type game, and they release it for full price, is it worth the money? 

Sounds exactly like the transition to RE5 from RE4 to me, streamlining the invitory, making a non co-op series co-op, taking out any and all scare moments to make action scenes that are on rails shooting people on motorcycles... hrm... 

As for the OP topic, I feel inbreeding is one of the worst things, making things far too complex isn't fun for the core consumer nor the general consumer, very rarely is it done right anyway, Monster Hunter Tri is the best in the series, still very complex but things are far more intuitive.  

I guess that's probably the thing designers overlook, people don't want more complex, nor dumbed down, they want fun and intuitive, simplistic things are often intuitive, simplistic things aren't always fun, IE platformers are pretty simplistic, not all are fun cause the level and control design has to be very well done, which is a complex process.  World of Warcraft is complex, how to get into it however is actually simple and intuitive, it's just designed very well.  

So basically all are pretty bad for a series, if a designer truly wants to improve the series they should make things more intuitive while retaining the level design, controls, AI, etc that made the game fun in the first place.

Deviation is definitely the worst.

Can't say much about inbreeding and casualising, except that I don't mind as long as it doesn't stray of too far.

Tiny upgrades don't bother me that much, only if they're useless or annoying.