Firstly, you're focusing on the power number while completely forgetting about weight. The whole point of power is to move weight, and less weight means you need less power. The GR86 is only ~2800lbs, and that 228 HP is good for a 0-60 that's consistently ~6s if you're rowing your own gears. Unlike the first gen, it has a better P/W (and weight-adjusted torque) than a Miata. Almost the exact same P/W ratio as a VB WRX too, and I don't see people complaining about how horrifically underpowered that car is.

Secondly, the GRC is NOT "the same money" as a GR86. It starts at over 40k, while the GR86 starts at just over 30. You can cross-shop a brand-new GR86 and a Civic Sport Hybrid at the same price. Good luck doing that with a GRC. It's a smaller price increase to go from a GRC to a Type R or 400Z than it is to go from the GR86 to the GRC. And that rumored 400 HP unit? If it's true, than I guarantee you it's going to make the GRC heavier and more expensive.

Why keep the same engine?

I think you're underestimating the cost and complexity of even fairly "simple" changes. Subaru/Toyota already decided against dropping the turbocharged FA24F into the GR86/BRZ for the second gen. Even just that would likely require some fairly significant redesigning to handle the extra weight, space, complexity, and power of the turbo, while the NA variant of the FA24 almost drops straight in. The GRC's G16E-GTS? Forget about it. At best, you're looking at a total redesign of the front end, and realistically you're actually redesigning the entire car once you've handled all the implications of a new engine and drivetrain, likely costing hundreds of millions of dollars in engineering, retooling, etc. On top of all that, if they did it now, they'd have to get all the safety and emissions recertified, also costing millions of dollars. It's already a niche platform, and the added power wouldn't change that enough to actually justify the cost of doing so. When you adjust for inflation, the 2nd gen is actually cheaper than the 1st gen. I'm willing to bet that they were only able to do that because they kept development costs low.

Finally, POWERRRRR and straight-line performance is not the point of the GR86. It's the classic sports car formula: NA, RWD, comes in manual, and handles like a dream. Giving it a bit more horsepower while losing practically all of what makes it special would be an incredibly expensive mistake.

I have one and I love it. I cross-shopped with the Civic Si and Elantra N, and the GR86 absolutely stole my heart on the first test-drive. The EN was technically faster in a straight line, but it just didn't feel special or handle like the 86 did. (I didn't even get around to testing an Si because the dealers were asking for a deposit to even test-drive one, and I wasn't playing that game.) My 86 is absolutely sublime on the curvy, hilly backroads in my area. Even in my daily commute, rowing through the gears still puts a smile on my face.

I’m wondering if there will even be any appetite for this aging platform for much longer

The GR86 is outselling the GRC before you even factor in sales of the BRZ. It's doing fine.

seriously, this platform is getting to “new Prelude” levels of unserious

The new prelude is getting panned because it has almost 10% less HP than the 86 while weighing over 15% more at an awful price. It has comparable performance to the 2013-era FR-S (which admittedly was a bit lacking), while being a FWD eCVT Hybrid, i.e. none of what kept the underpowered FR-S compelling. Oh, and it costs almost 50% more than a base GR86 does BEFORE you factor in dealer markups. Price-wise, it's competing with the likes of the Type R, Supra, and 400Z. The GR86 is mostly competing with the Civic Si and Miata. Totally different market, and it shows in public reception.

It's a great car, and in the real world the power is more than enough. It always puts a smile on my face, and I was able to afford it comfortably.