speedlever : I’m glad you are sticking to what you like with your 6603 Ultegra. I wore out the 39 tooth on mine and it was beyond shark tooth. Mine had a wide U shape with a flat gullet at the bottom. Made my new chain disengage while standing on the pedals, very dangerous since it happened in traffic when starting from a dead stop at traffic lights. My 6603 crank had ~16,000 miles on it at the time. I was able to locate a replacement (last year) from “Mike’s Bikes” in California. I checked back after I bought one and decided I needed more only to find that they are sold out now.
I’m with you about the beloved gear display (Flight Deck) being something you don’t want to give up on. If Shimano would listen and bring back a road triple group with these exact ratios on the crankset, I suspect they would sell well.
One thing to consider about the middle 39 tooth ring is that it is pretty close for a 39 tooth 130mm BCD chainring for a double. It is just missing the shifting ramps for downshifting to the 30 and upshifting to the 52. So I suspect you could run your 6603 “forever” just by replacing the rings with ones that are of the correct BCD. But I’m like you and the idea that you can’t get a matched triple set for these cranks kind of stinks.
In your searching for replacement parts on eBay, you might also search for “6604” cranksets or chainrings. This was the same as the silver 6603 but in a limited production champagne brown anodized color. About a year ago there were several sellers selling complete NOS 6603 or 6604 cranks for ~$150.
It occurred to me that an aftermarket chainring manufacturer could step in and produce a matched set of triple road rings for Shimano. I emailed Wickworks a while back to see if they made this item and they replied that they did for a short time but that they were discontinued because they didn’t sell. But they had no old stock of them either. Wickworks matches their chainrings and they have their own version of pins and ramps to quicken upshifts and downshifts. We need a crowdfunding group to prompt a machine shop to CNC mill or even 3D print these (somewhat) proprietary rings. It gets a bit exhausting on these forums when you say you like a particular configuration of gearing and then folks are minimizing your choice lecturing you that something else is “just as good”.