OP is presenting a strawman argument. We already know performance of pre-production models thanks to Linus Tech Tips. It runs DOOM Eternal at 1280x800, Medium Settings, at roughly 60fps avg. In like-for-like comparison it is 59% faster than Aya Neo, which runs $925-1015, while Steam Deck runs $399-649. Nobody has claimed it's anything but that.
Don't get me wrong, Aya Neo and GPD Win 3 seem like great devices, but they are upwards of 2-3x more expensive. Valve stated that they designed the Steam Deck to be the physical manifestation of your Steam library. I have over 300 Steam games, and over a hundred more via GoG, Origin, and Ubisoft Connect. This is excluding Game Pass and emulation. That's thousands of games I know I can play. As more games support FSR and as the FSR Vulkan hack for Proton is refined, the performance of games on the unit that support it will greatly improve. I made a quick video showing the potential of FSR on even meager hardware like Ryzen 2400...
1280x800, FSR 'Off', High Settings: 43.80fps avg | 36.53 low 1% | 30.65 low .1%
1280x800, FSR 'Quality', High Settings: 63.88fps avg | 53.47 low 1% | 44.36 low .1%
1280x800, FSR 'Balanced', High Settings: 70.88fps avg | 58.80 low 1% | 48.52 low .1%