It’s not like the piston doesn’t move “straight up and down” as compared to the cylinder walls in a V. And in an I motor the connecting rod journals are obviously still offset from the mains, so the connecting rods will be at an angle compared to the cylinder walls at all times other than TDC/BDC just like a V. There is still side-loading on the pistons/rings etc. That has absolutely nothing to do w/ being an I, V, W, Boxer or whatnot.
Arguing that an I configuration is more compact is also ridiculous. It all depends on the exact application. Sure, in a FWD platform with a transversely mounted motor an I layout allows for a shorter engine bay and would be considered more compact. Since overall width is dictated by the desired vehicle track-width and passenger compartment an I4 will easily fit. But what if you want a 6 in a FWD transverse engine platform? Well then a V6 would be considered “more compact” and much more practical to package.
Further, RPM capability has little to do with layout. In general RPM capability is dictated by the reciprocating assembly weight/strength, valve-train weight/ability to control valve motion, combustion chamber efficiency, and the efficiency of the supporting systems to supply enough fuel/air and also expel the exhaust gasses.
You also state “most are hemisphearicle designed” (your spelling, not the dictionary’s 
) but that is entirely independent of the cylinder layout. IF it just so happens that a lot of I engines were designed with hemispherical combustion chambers that is just a correlation. It does not represent an inherent benefit of the cylinder layout itself.
And Gravity? Really!?! GRAVITY
Before we get into other reasons that is entirely stupid, in a V engine gravity would still be acting on the parts within the engine. Just because they don’t travel directly perpendicular to the horizon does not mean gravity does not act upon them. In a V gravity’s force would end up being distributed between friction between the pistons/rings/cylinder wall and force along the vector of the piston’s travel.
BUT beyond all of that the force of gravity is absolutely minuscule when compared to the power generated by a modern internal combustion engine.
And FINALLY, while one piston in an engine is traveling downward there is likely another traveling upward which would be FIGHTING gravity in an exactly equal manner.
Gravity. Nice one.
So what are the advantages of an Inline configuration?
Simplicity is probably the primary benefit. Assume the motor is going to use an Overhead Cam configuration you only have to drive a cam or cams at the top of one head. With a V configuration you end up with much more valvetrain complexity and mass. You also only have to worry about getting air in/out of one head.
WTF are you even babbling about??? Seriously.
Please explain how an Inline cylinder layout promotes a broader torque curve. I generally like to attribute the characteristics of a given engine’s power curve to factors such as displacement, valve events, maximum RPM capabilities (as discussed above max RPM is largely independent of cylinder layout) and the ability of the supporting systems (intake/exhaust primarily) to support the engine.
If you’re going to make claims such as inline motors having “a nice flat torque curve” please have some sort of reasoning and/or evidence to back up your ascertain. And how would you explain the prairie flat torque curves of motors like the Viper V10 or the LSX series if flat torque curves are a property unique to inline configurations, and therefore one of their benefits?
-TJ