First- Number of bearings-early straight sixes had four mains( one at each end and two dividing the six pistions into groups of two). Later ones had seven mains- a main between each cylinder and and the ends.
Second- Imagine a crank with ONE main and six cylinders. Have to be a BIG main and a sturdy crank! Each additional Main lowers the load on each bearing.
Next rpm. Don't know the exact math on rpm vs load(i. e. life), but it is not proportional. Probably, square or cube. One of you academic types can answer(I've forgotten). Running at 4000 rpm or 80 mph is about all we Americans can do, IF we can find an open highway. Most driving in Boston is 0-15 mph! But, an occasionsal venture into 5000 rpm territory - OK. 7000 rpm? why? how? when? How many hours will it last at 7000 rpm vs. 3500 rpm? You'll get there twice as fast, but only 1/5 or 1/10 as many times. Your choice.
So, rev it up to 7000 and listen to the timing chain, chain guides, camshaft, valves, rocker arms, rings, etc. SING big time. Metric, Gernod, here it comes.