8yrs for a seedling purple/green is not that unusual. Heck, some grafteds can take 4-5. If it was a pink I'd be worried!
The other option is to prune back super hard, fertilise with glyphosate and plant a new tree grafted from the other fruiting ones you have, or stump the big one and graft the shoots.
A friend has one that he keeps well pruned, but still stands about 6m tall and at least that wide, which often gives heavy crops of small seedless fruit in addition to the regular ones.