Akin to you building a kit car or home-built aircraft in your garage. Ya you can build a one off, fly it, but then truly test fly it, but now enter serial production with suppliers, material certs, the processes held too and the parts inspected to print, then assemble and test.
That is where the rubber meets the road and with the Ruskie's track record on their engines, ya it might work if we gave them some Pratt F100-220's other than that, you maybe spot on. This maybe another situation like when we thought the MIG-25 Foxbat was unobtainium, and it turned out to be a lead sled...
Oh yeah... I remember all the hype about the mysterious super-fast Foxbat. The Warp-9 Superduper Ruski-Klingon suborbital battlestar that was gonna kill us all.
And then Viktor Belenko defected with one and we found out that it used vacuum tubes in its electronics, weighed a huge amount more than expected, and that every time it hit the high speed we thought was in its operational envelope, the engines had to be replaced when it landed.
I suspect this new bird is not going to be that bad. Let’s face it, it’s a lot easier for the Russians to buy tech these days than it used to be, but still, I do not expect to see the PAK-FA in operation in significant numbers before 2020.