Question

Hello and blessings,
In many cases, there are halakhic rulings whose rationale has been lost or is no longer relevant, according to all opinions, in our time.
In addition, there are customs that sometimes directly contradict the Jewish law as ruled in the Talmud / Shulchan Arukh, etc.
My question is: do these rulings and customs still obligate us today?
The claim is well known that when the reason falls away, the decree is not thereby nullified; but it seems to me that this too is a rationale that has lost its meaning, since it was established at a time when there was a Sanhedrin, and the underlying assumption is that even if the public thinks the reason for a certain law has fallen away—if the Sanhedrin did not revoke the decree, then apparently the reason has not truly fallen away. But today there is no Sanhedrin at all to revoke decrees, and so we are “stuck” with arbitrary rulings that we cannot change.