The main mistake with the Sega CD was adding a bunch of expensive new hardware to it. It gave the Genesis a second, faster Motorola 68k and a special ASIC to handle sprite scaling and rotation effects (to compete with the SNES), but these inclusions effectively doubled the price of the add-on and were seldom used by developers. The Turbo-Grafx CD, on the other hand, just added a CD-ROM drive to the Turbo and cost half as much. The Turbo CD was also much more necessary since the console used HuCards that were much more limited in space than the cartridge ROMs used by the Genesis.
That said, the Sega CD was reasonably successful for what it was, and I don't think it contributed much to Sega's downfall (if anything, it helped Sega foster an impression with customers of being on the cutting edge of things back then). Far more egregious were the 32X, since it confused customers about the company's direction after the Genesis and was abandoned so quickly, and the Saturn's architecture being so much more expensive and complicated than the PlayStation, especially for 3D games. The cost let Sony undercut them because they knew Sega couldn't match them without losing money on each console, and the complexity was such a hurdle that even Sega's first-party developers had trouble figuring it out (they had to release "do-over" ports of Virtua Fighter and Daytona USA because the first versions were so rushed and buggy). Sega of America also pulled the plug on the Saturn way too early (because of Bernie Stolar's infamous interview where he said the console wasn't the company's future, which caused third parties to immediately cancel their releases for it), which left the company with basically no presence in their biggest market for about two years.
I made a few simple Mac apps in Objective-C during the pandemic in 2020, and I was struck by how hard it was to find tutorials and examples on how to do anything in Objective-C. It's possible I was looking in all the wrong places, but it seems like everyone assumes you're using Swift at this point. I would advise against learning Mac development with Objective-C except to people who have a specific interest in it or need for it. I also think most people accustomed to newer programming languages would probably find it arcane in some areas and tedious in many others (although I personally like it for what it is and don't regret the time I spent with it, I definitely felt like I was making things harder on myself by using it).