Couple things mentioned in here...

Realoem is dead in the water. It hasn't been updated after 2020 if I'm not mistaken. ETK is the only reliable, up to date, parts source which I have a dealer login for. I stopped using Realoem 2 years ago due to it's constant ads & issues with new model parts.

In regards to the E53 being relegated to BMW Classic. My opinion is they will keep producing parts like they currently are due to the high demand of parts needing availability. This is due to due the high number of these cars still on the road across the world. While it might be difficult to get specific unique parts, the common ones will be around for a long long time. The same applies to the E46 as well. Regardless, this back and forth about parts quality is going in circles. BMW OEM parts are still the highest quality parts you can buy for your BMW. There are excellent examples of OE or Aftermarket parts being produced & while they are cheaper and sometimes work just as good, you should always consider BMW OEM first when you want high quality.