Most products will bead intially because you are leaving a smooth slick surface...I don't know too many people that would throw a coat of wax over a dirty rough car , thus the use of car washes, clays, swirl removers , polishes, and compounds to clean and prepare a surface. New cars are relatively smooth and sometimes a wash will prepare it enough for a quick AIO product that dealers or their detailers use.
The better the surface is prepared , the better the beading will be. For many years when i was cleaning my cars on a daily basis, I only polished my cars, like many of the Street Rodders I dealt with...no wax ..no sealants..nothing ....my paint always shined and always beaded.
Now if you do use a wax or a sealant, and especially a wax, beading will vary by the heat and cold, it will vary upon how dirty the car is, reducing surface tension, and also as the oils in the carnauba or other types of waxes diminishes.
Sealants on the other hand will usually either bead, or sheet after beading. They are more slick by nature if applied and cured properly, but they will also be effected by surface dirt and contaminates in the air.
Now to your point, if you keep your car immaculate and highly waxed, and the car is clean and the beading during an average rain is not the same as it was right after waxing, yes you are seeing the diminishing strength of the wax or sealant.