During Brawl, the sky was the limit. People would talk about just any character, no matter how likely or unlikely. It was fun and exciting.
Smash 4? When it became obvious relevance > all, it became a game of “who predicts Nintendo’s marketing pattern is the most correct.” Popular characters were looked down upon if they didn’t fit into Nintendo’s bottom line that year. It became about guesses and predictions, not discussing the merits of individual characters.
K. Rool was a splash of cold water to a lot of these people, but one look at ResetERA will inform you this mindset very much still exists.
Speculation has indeed improved tremendously.
As this post points out, if it weren't for the ballot and K. rool, Smash speculation would have been so one-dimensional and kinda sterile as it would have boiled down to what's recent within the last couple of years with the exception of a "retro" rep, which will always be only counted if they are from the 80's and before.
People have started using "old character" pejoratively against the likes of Isaac, K. Rool, Geno, etc. and it perplexes me why the era a character hails from should matter at all. Why would lacking a recent appearance make them a poor choice if they're a fan favourite? Did Smash 4 brainwash you that hard?
People delude themselves into thinking that having a recent appearance is the be-all-end-all when it comes to fan appeal and their perception of importance is really skewed towards what's recent or what's arbitrarily counted as a "retro" rep (despite the concept of what is actually retro not being limited to the 80's and before). That's why you'll rarely see characters like Mach Rider be called irrelevant.
At any rate, even if Isaac isn't all that well-known, he definitely has something that people disregard - the Roy factor. What I mean is that thanks to his frankly cool-looking and diverse powers as well as generally representing something new and different (as opposed to being another Fire Emblem or Xenoblade character), coming from an unrepresented franchise, it's easy to make a great trailer for him that will impress and endear him to even people who have no idea who he is.
Before Brawl, it was Sonic.
Before Smash 4, it was Mewtwo
Before E3, it was Ridley.
Before the Direct, it was K. Rool.
And now, it's (apparently) Isaac.
People have been so used to making the overall most popular character request at the given time a hate sink, that whenever it's taken away (i.e., made playable) they'll replace it with the next most popular character. A desperate, cyclical desire to fill a void that they've grown accustomed to.
People were hating on Sonic during Brawl? I wouldn't be surprised. Honestly speaking, given Sonic could have easily not made it into Brawl if Sakurai didn't delay the game for him, I'm so happy that he made it in.