“Basically, the secular calendar is sliding forward slowly, by 3 days every 400 year”
This has been mentioned previously.
In 400 years the date for vesein tal umatar will be Dec 7/8 in 4000 years IT will be in January (I’m rounding). given enough time We will start saying vesein tal umatar after PEsach. now of course Tekufas Rav Ada isnt perfect either and PEsach is slowly sliding forward as well (this received alot of attention a few years back when the first day of chanukah occurred on November 28 (Thanksgiving) which wont happen again for some 70,000 years until chanukah passes through January, February, MArch etc until it occurs in November again ( note: it will occur on Nov 28 a few more times until 2146 which will be the last time for over 75000 years but none of those times are a thursday). however given that Tekufas Shmuel is less accurate it is sliding forward at a faster rate 3 days in 400 years (as mentioned) while Tekufas Rav Ada slides forward approximately 1 day in 240 years.
In both cases chazal sacrificed accuracy for simplicity and Long before it poses any real problems we will have a sanhedrin to fix it. (OR reinstitute a calendar al pi reiyah)