HEVC breaks the video quality of 4k

What did you intend to do?

• H.265 is HEVC (similar to ‘H.264 is AVC’).
• You second (lower) screen shows H.264, not H.265.
• The data rate (bitrate) is the file size divided by the duration. The smaller file will always have a lesser data rate, for the same duration.
• Converted files with lossy codecs will always have generation loss. Each export has worse quality than the one is was exported from.
• Converting to reduce file size will compromise quality. The trade-off should be apparent. Even with H.264 to HEVC.
• H.265 has more efficiency (quality for file size) than H.264. Side-by-side comparison shows conversion to HEVC and conversion to H.264 should get about 1:2 file size ratio, meaning HEVC can be roughly twice as efficient. Your conversion is pushing it much more (1:4.7), losing more details in the process.

(Edit: I may have mixed up ‘source’ and ‘export’ initially.)

What did you use for conversion? QuickTime Player?

Your specific content may need much more control over export settings, including bitrate. There a a lot of parameters for HEVC, and QuickTime simplifies/‘helps’, by not showing all the switches and values that are used.

Your content may need a conversion program with more detailed settings for export.

What if still HEVC video can't provide same build quality up-to H.264 video?

Let me try a sample. And then you can see for yourself. I found the source, and I’m getting it now.

If it does-not provide same quality then why would people how prefer quality as their 1st priority prefer to use this feature?

This is not about H.265 vs. H.264, but about ‘A’→‘lossy copy of A’. The ‘generation loss’ argument still holds: a copy of a copy is less than the original. H.264 to re-encoded H.264 would be even worse than H.264 to H.265.

H.265 makes 4K streaming more accessible, as 15–25 Mbps is more available than 30–50 Mbps per device. And cheaper too, as people are paying for bandwidth, even if averaged out in large groups.