Soon Zen 6 should become available (well, or not so soon). With 7200 MTs memory controller? I haven't seen such JEDEC modules at all. Are they possible without the additional clock driver (like on 6400 CUDIMMs)?
Speed has indeed decreased for the modules you can actually buy, while historically you would get faster and faster modules at lower prices in the years after the introduction is a new DDR standard.
No end in sight.