Really? (1) Do you have a cite for that (perhaps, since "optimal" is in quotes)? I've heard people say that 3 times a week is enough to make steady progress, not that it's "optimal".
(2) Right, but is that a good analogy? People don't get hurt playing the cello, and if a rank amateur cello player should misrepresent her skill level and get into a performance of the London Symphony Orchestra, she's not going to end up in the hospital as a consequence. And what's the analog in martial arts terms of having a jam session with a semi-pro? Getting into a bar fight?
(3) I doubt that the average Okinawan farmer practiced anything but farming and sleeping for two hours a day every day.
(4) I think that's good advice; however, considerations of spouse, family, career and basic play time doesn't leave a great deal of time left over. For most of us who are not rich and can't afford a maid to do all the cooking and cleaning, a handyman to fix everything that breaks around the house, and a nanny to look after the children, normal people in normal situations where we cannot simply dump off our responsibilities onto others, two hours a day every day is simply not feasible.
(1) yes: http://www.aikiweb.com/training/goldfield1.html
"...According to the findings, 3 training sessions (of 1-2 hours duration each) per week is the most productive
in terms of both physical conditioning and improving one's skills. ..."
(2) feh, good enough to make a point that aiki and cello are skills intensive, not as much muscle power intensive. What, we can get hurt in aikido:
cool! (...just kidding, just in case you are too serious) ...and a fight at a bar's ugly & scary... do not want to get in that (again).
(3) that's what good old late Gichin sensei wrote, but I will not quote him
(4) true true, thanks, but we do what we can balance. I will admit to hiring contractors for occasional upkeep services and while we were raising our son I did have years of laps in regular training...