The Dory

[PMBardunias]

I think you misunderstand my point. One limitation on how long a normal spear you can wield in phalanx is that for every foot you gain forward, it is counter-balanced by an equal foot backward - which interferes with ranks behind you - not to mention becoming heavier.Thus 7-8 ft is the practical limit to length. A tapered, rear weighted spear allows one to have, say , a 9-10 ft spear, with still only 3 ft protruding behind you.

I don't think this is such an obstacle. If held overhand ant even a very shallow down-angle, then the first few feet are the most critical in hitting the head of a man behind you. The same is largely true for underhand grips as well. If you can hold a 12 foot spear with one hand, you could surely use it/

The ultimate expression/development of this design trend was the pike/sarissa which allowed a length of 18 ft or so, yet with still only some 3-4 ft protruding behind.Thus the changed balance point/tapering allowed the production of longer spears, leading to the longest possible practical spear - the pike/sarissa.

This has very little do do with tapering and back-weighting. Surely you don't think a 18' sarissa was balanced at 3' from the rear??? The sarissa is a two handed spear and that changes everything. The balance point should be at or ahead of the left handed fore-grip. A two handed pike can be gripped so that no shaft extends beyond the rear hand- as in early modern pikes.

When you take the rear-weighted balance to shorten back-length concept to its logical conclusion we end up with a sword- a very heavy sword.
.....nope! You end up with a pike/sarissa !! A sword is a very different weapon with characteristics altogether different. The only factor in common is the rear-wighted balance point.

A stabbing sword is a essentially a very light, short spear with the balance at the hilt caused by the heavy pommel. The two handed sarissa is not a direct outgrowth of the rear-weighted, one handed dory. In fact the opposite. If sarissa were tapered and rear weighted, then it is a parallel evolution of a simple, non-tapered pike to a tapered form that had the advantages described above. The whole point of the taper and weighting system is to NOT make the spear any longer. A two handed spear may have evolved as a counter to the rear-weighted dory, because they were probably hitting the limit to how long a one handed spear can extend beyond the grip. To get a spear longer than about 8' tapered, 12' untapered, you need two hands I'm guessing.