With SEGA being a third party video game company, Sonic isn't absolutely necessary. I'd even argue that mascots are indeed a dated concept as Sony and Microsoft don't exactly flaunt mascots nowadays. However, I think Sonic has been a mascot for so long that to suddenly drop him would be a bad move on SEGA's part. While there are gamers out there who see Sonic as being a series that is "mediocre to terrible", there are also the general public, casual gamers, and a large fanbase who just plain like Sonic and when they see him they immediately think of the good times.
In my opinion, Sonic is essentially an extension of the SEGA logo. You see SEGA, you think SEGA. You see Sonic, you think SEGA. It still works, so why change it up.
Sonic The Hedgehog is what helped give SEGA an identity. He's to SEGA, what Mickey Mouse is to Disney. He's an Icon. He'll never go away.
Sonic The Hedgehog the main series itself is now under Sega of Japan's complete control and management. I think this a much more sound and proper approach since SOJ never really influenced or managed him. It was always Sega of America who did. And now they're handling Sonic Boom.
As for the mediocre quality games, the reason is simple: He doesn't belong on non Sega home platforms. Period. That's like saying Mario Galaxy would be fun on PS4. Back in the Early 2000s, keeping the franchise going forward was a necessity, and Sega becoming more software focused was a vital move that was needed to save them.(It's now a complete necessity.).
Sonic was created specifically for FAST paced, High Speed gameplay in which was duplicated using Sega's hardware schematics.(Mega Drive had a fast CPU so its Sonic games exploited that) What made him so unique and special was that each title was developed for a specific SEGA console. Completely from the ground up. I'm sure Sonic Pool would have be something special had Peter Morewic gotten STI on board and encouraged them to drop the doomed 32X/Mars' developed Sonic Mars/Xtreme.
Sonic 2006's tragedy is that it was 360 exclusive at first, and then Sony licensed it for PS3 giving Sega little to no time or experience to properly code and program the game for PS3 a NEW yet to be launched platform at the time. It probably wouldn't have turned out so bad, had it remained exclusive to 360.
People keep whining and moaning over ROL, without understanding that BigRedButton started development of it on Wii U from the ground up, but didn't understand how to code a proper engine, nor did they have any inexperience working with slower hardware. To be fair, the fans ASKED that someone else besides ST develop a game and they got what they asked for.
Notice how everytime SEGA tries to develop a Sonic game from the ground up on other consoles, it always falls short no matter what they do, even porting to various consoles seems to always have mixed results.
The Reason why Sonic Dash and Sonic Runners have been so successful is that they have the right audience and demographic as well as a much more broad market that's easier to please. Console Gamers have gotten so picky,double minded and overzealous, one minor issue would cause them to start spewing hyperbole and anti Sonic rhetoric. The sad thing is that, during Sonic Heroes, people whined and complained that he had too many characters, and then by Generations, they started whining that you can't play as other characters.