It's not even the fact that they really enjoy Sonic and are excited for more Sonic.
It's that Sonic games don't have a great track record for being very good outside a couple exceptions. So the idea that the fanbase is still so crazy about Sonic despite the lack of stellar quality is just wild to me.
I feel the same thing about sports fans of teams that never win anything.
-YMMV as to what counts as "very good," but there's definitely more than a couple who fall into that territory. This isn't me being a fanboy, I'll happily tear apart something like Forces, but bear in mind that Mania released the same year as Forces, and Frontiers was definitely a "good," if not "great" game.
-That said, I actually agree it's weird that Sonic's managed to still keep going despite a number of, to put it mildly, fuck ups (06, Sonic Boom, etc.). There's certainly more consistent platformer series out there that have fallen by the wayside (Crash, Spyro, Sly), and while the first two had a decline after their original trilogies, they just fizzled out while Sonic just kept going, even if it had more spectacular crashes. I'm going to posit the reason as to why, namely that the reason the Sonic games have gone on is that the IP as a whole has had immense success elsewhere. As in, regardless of one's personal feelings, here's some facts:
-The Archie Sonic series lasted 290 issues (plus spinoff series, specials, etc.) making it the longest running comic based on a game in history
-Sonic Boom (the game) crashed and burned, Sonic Boom (the cartoon) is generally well regarded - there's a reason why no-one cares about Lyric (game), but everyone likes Sticks (cartoon) for instance
-Looking at the Sonic IP as it currently stands outside the games, how do things stand? Well, the IDW comic series is still going strong (over 60 issues at this time of writing, plus numerous mini-series), we've had two successful (both financially and critically) movies with a third on the way, along with Sonic Prime (with more coming), plus the upcoming Knuckles spin-off series.
So basically, I'd say the reason Sonic's still around as an IP is that the IP as a whole is strong, despite the games themselves being hit or miss, everything else has been consistent. Regardless of how one personally feels, the IP's probably at its strongest point in ages.