^ Gonna continue on from this since I watched a lot of the CGI era bloom.
The CGI stuff was pretty bad mostly because of the poor writing (head writer at the time was Sharon Miller so this is referred to as the Miller era). They had this rhyming thing going on, characters were over-simplified and reduced to one defining trait, etc. A few years later, around the time Mattel took over T&F, Andrew Brenner became the head writer for the show. Brenner understood the source material that was the Railway Series books written by the Rev. Awdry and also engaged with older fans to see what they wanted from the show. This resulted in the series taking a slightly more mature approach that also appealed to older fans as well as kids, actually showed a more realistic running of a railway, linked back to the RWS, and brought in interesting new characters as well as bringing back old fan favourites from the RWS and model eras.
It was clear that Mattel, especially later on, made the writers and animators plug in some really weird ideas because they wanted to market new toys. A lot of these ideas were put into flashbacks or fantasy scenes because they didn't fit the element of realism that T&F had. It got more obvious towards the end of the Brenner Era.
Then... Mattel decided to revamp the show again because the franchise was starting to lose money, more because their marketing sucked but they placed the blame on the show. Big World, Big Adventures happened. Brenner left the show and it was remade into a series that was half based in Sodor, half based on Thomas visiting different countries. They also wanted to bring in more gender equality, and so they pretty much wrote Edward and Henry to move out of Tidmouth Sheds and pretty much out of the show in general. Keeping in mind, Edward and Henry are two of the original three characters that appeared in the very first RWS book (the other being Gordon). Fans, understandably, didn't like this and saw it as disrespectful to the source material, especially as the female characters they were replaced with were so dull. One was a bright orange engine called Nia whose whole personality was, "I'm from Kenya," and the other was a bright yellow express engine called Rebecca who also didn't have a lot to her. It's not like they needed to do this; the series has plenty of different and complex female characters who could easily be written into lead roles for a few episodes. The going around the world stuff didn't work either because most interpretations of those countries were so stereotypical they were borderline racist.
Meanwhile Mattel continued to lose money. So the logical thing to do, of course, was the make T&F into every other generic pre-schooler show. Hence, All Engines Go. It is so far removed from the original source material that I do sympathise with the fans, both young and old. This isn't Thomas. The media has now run a story saying how much of a disgrace it is, parents are commenting on their social media saying their kids hate it, and the fandom on Twitter is still pretty outraged. Nobody likes this.
I think what drew so many people to the original show was, despite being a world where the railway engines talk, it still gave kids a pretty realistic introduction to how the railways worked. A lot of the accidents were based on real life events. Pretty much all of the engines are based on actual locomotives and the Reverend made a complex history for them that tied into actual UK railway history. The characters all had different and interesting personalities. The model series also had a darkness to it about the events of the 60's when steam engines were scrapped en masse and whilst Sodor didn't seem to be moving with the times, if you didn't keep the fat controller happy, you could also face the smelter's yard. T&F also has a good relationship with heritage railways here in the UK, and I'd say it has helped bring in the younger generation to help keep the railway preservation efforts alive. This show really was something special, so I'm not surprised at how furious some people are with the reboot. It's pretty sad, really.
