Archiving final video, exporting each frame as PNG
 

So I gave this a try a like the results let me know if it's a good way or not so good.

I had a music video, and need to Archive, so Instead of making a video file, I exported the sequence as PNG files and the audio separate.

My plan is then to store files on a "cloud"
When I need it I can just download and bring into editing and export as needed.

Re: Archiving final video, exporting each frame as PNG
 

Depending on the actual png your editor makes, this is a lossless method, yes. However, more work to create, more files to manage, fragile to damage, and more work to reconstruct to video.

I may be missing something in your requirements, but more common archive methods would involve mastering to lossless or near-lossless video codecs. Such as DNxHD, Cineform, QT-Animation, MXF, etc.

Re: Archiving final video, exporting each frame as PNG
 

Conversion between BT709 and RGB color may not be completely lossless. For long term archival, I would suggest a SMTPE standard video format with multivendor support.