Methane decaying just means the atmospheric concentration doesn't increase as fast. But if you emit more than the previous year then levels will go up. And indeed, levels are currently 3 times what they were from pre-industrial levels [0].

The EPA says its Global Warming Potential over 100 years is 28-36 times CO2 [1]. 100 years sounds like a very relevant time period to me.

[0] https://en.wikipedia.org/wiki/Atmospheric_methane#Patterns_o...

[1] https://www.epa.gov/ghgemissions/understanding-global-warmin...