Being an owner of several big Onkyo AVRs, I don't know of any case, where a ventilated AVR failed if this had been done right from the beginning. Most of the time ventilation has been added only after the first problems occurred. Those units were often been mounted inside a cabinet, where heat especially becomes a problem.
It is known, that the older Onkyos became quite hot, whilst the latest generation does not seem to have this problem.
But it stays undisputed, that some of the older units starting with the generation xx5 had heat related problems which led to the early failures of the HDMI board if not handled preventive from the beginning.
I am still running a (ventilated) 876 in one of my rooms, which works the same like from the beginning. But other had different experiences.
It is not known yet if the generation xxx9 and xx10 still have any heat related problems, because they are still to "young" for anything meaningful.