Whilst the older worlds may be slow, the new worlds also have either forgotten attacks or willingness introduced. (Now worlds have both.)
Where you progress to when your bases (all 31) have reached level 65 and a little beyond is more of a pressing issue I feel. (I've not reached that point, but many on the old worlds have!)
I agree they need refreshing, but not in the way you suggest. Original worlds didn't have a fortress or the current bonus system either so they have been refreshed in the past.
As this is in the suggestions section, I'd add a suggestion. Allow the worlds to have bases bigger than the current upper limit than 65 - maybe 105. We saw on early pte's they could reach that size and although other things were causing it, they did not appear to totally break everything.
Causing the forgotten to spawn above the current lower limit would also be helpful.