I'm sorry I'm just going to correct the spelling first, it's
Acalypha with a "y" - and that might help you with any research you do.
Can't turn up anything on life expectancy - I think you've probably done pretty well given (as you say) both drought and flood conditions in SEQ over those years.
Perhaps the root ball just grows woody as well over time and possibly with those climate extremes lays it open to invasion by anything from termites to borers. Maybe the soil suffered with those conditions and formed too difficult a barrier for new root growth.
I'm glad you took cuttings, they are such attractive plants and often regarded as "pass along" as you don;t see so much varieties of the old large leafed ones.
Perhaps renew the soil a bit and try to get rid of any old bits of root boles before starting over with your borders

I have a lot of Acalyphas which grow as a dense thicket. They were well established when I bought the place and I've had it for just over 10 years. Each year I prune them back savagely to short sticks. The prunings I chop up and use as mulch around the plants. The mulch is very deep. They're doing quite well but I've often wondered the same, whether they have a "use by date". They've never had any fertiliser, so perhaps I should start doing that.

I would consider it Tropic, the soil we are on is lacking all nutrients and that could have been one of my problems.

I have been leaning towards the idea of less fertiliser means less pruning and trips to the dump. Changing my mind now and aimng for healthy plants for a little outlay.

Nod