The connection between half life and decay energy isn't so linear. Looking at a table of gamma ray energies [0] one can see that the lowest-energy distinct five have half lives in hours:

    5994
     104
       6
    2904
  482117

with the highest-energy distinct five in hours:

          12.8
          15.0
           3.48
           1.9
  6280920000    (717,000 years!)

However since the output power is the product of the activity and the decay energy it certainly follows a diminishing curve since fewer decays as time advances.

[0] https://application.wiley-vch.de/books/info/0-471-35633-6/to...