Being stronger is actually a skill, not all Europeans hit the same km/h when you can hit +100km/h that's sure a skill. Being taller itself is not a skill however taller people usually have a higher spike reach which makes them able to spike better and usually they reach the block better/easier as well, this all ofc unless you're called Mireya Luis.
Foreigners improve the league a 100%, a lot of these foreigners teams beat the Japanese at Wch or even Club Wch which means some are 'better' than them. The Japanese learn a lot of skills actually from the foreigners, their defensive skills only become better, they learn how to stop a Mihajlovic spike which is a lot harder than a Koga spike f.e. They will observe how a player like Akinradewo jumps and try to imitate that somewhat etc. etc. of course the foreigners improve the league as they bring something to the court that most Japanese can't and the locals end up having to adapt being still able to play well despite that. If it was only locals the Japanese NT would probably look like a bit better version of the Filipino one.
Though I highly doubt they change the amount of spectators.