I'm not too worried about Chinese versions unless they can do better quality at lower price.
Yes but these Chinese manufacturers are ok with very little profit margin, way less than anyone can handle. but as you said, early on they won't jump in.
About FPGA pricing, I'm not sure where people are getting ideas it'd cost more than $100 per unit.
you posted 2 models, and I checked them using Octopart (and distro sites like Digikey and mouser) and seem to give the prices as I posted earlier. Maybe you can get them cheaper directly from Manufacturer but still won't be too much difference especially if you calculate separate shipping and handling fees. I don't think you search for these stuff on Aliexpress. Notice that I posted prices based on small quantities due to not knowing the production batch quantity.
For ISL51002 I deliberately chose 165MHz version even though not everyone benefits from it. The price difference is not massive and otherwise the spec would be inferior to classic model.
There was ISL98001 which seems similar and a lot cheaper, but it doesn't give you sync measurements as much as ISL51002. Also, for ISL51002, the 165MHz seems the only supported version from manufacturer, the others are discontinued = bad long term choice, while 165MHz version is good choice.
ISL98001 has 170Mhz version which is 25-35$ while ISL51002 165MHz version is about 45-60$ or so. If you don't need sync measurement and can do them elsewhere, then ISL98001 should be better due to price.
ISL51002 doesn't have PAL/NTSC decoding, but it should be possible to run CVBS/S-video through its ADC and do decoding on FPGA
You would need something like TVP5146M2 on a separate module connected to your IO expansion board which delivers digital YC 20-bits signal, I assume your FPGA scaler IP handles 20-bit YC properly. if it handles 8-bit ITU-R output which is famous then getting cheaper ADCs is viable. Extra module shouldn't be costly for S-video and CVBS IMO.