I wouldn't necessarily say Internet forums are declining.
I'd say it's a combination of:
*Millennials getting older, busy with their careers and have settled down with kids/partners. Even during the hey dey, the weather boards were mostly a teen and early 20-something ordeal, a time when you have virtually no responsibilities, good health and are full of adrenaline. Gen Z and Alpha are also smaller in size than Millennials were (thus a smaller audience of young weather weenies as a whole).
*Society has become a lot more, socially, politically and economically polarized since the 2000s and early 2010s. So much of the community is now bifurcated into our own little echo chambers.