I do fully agree with you that arena design is the last, final big infrastructure investment. And yes, limitied resources are for sure an issue. However, me, when the time will come, I will go for the 4M design as it is clearly justified that in the (very) long run, the 4M arena gives you the highest return on investment.
Sure, if you might not plan to stay as long as it needs that return on invest to come back to your team, the lower arena designs might also be a good option. It depends on personal situation of the respective manager.