If you make an average of 5mil gold+elixir a day (an estimate amount of farming 2-3mil gold a day), it will take you approximately 158 days to finish the rest of legos and lavas. In 158 days you could also have maxed TH10 defence and higher level heroes (since DE is easier to get in TH10). Which do you prefer?
In my opinion the benefit of having maxed lavas instead of maxed legos is marginal. I'm only doing it because I'm a fairly active farmer and I can constantly upgrade at least one hero anyway. But if it will take you half a year and you don't enjoy farming walls that much, I don't think it's worth it to stay in TH9 for that long. It's not like it will take that much longer to finish them in TH10 (about 70%-80% as fast I heard).
EDIT: I excluded collector gains and other factors for simplicity, in reality you could probably finish faster than 158 days if you maintain the same rate everyday.




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