Those unused pins are usually there for mechanical purpose (strength/warp/crack) and have no signal or just grounding.

Typically they'll just change the pin-out i.e. pin A2 used to be VCC is now VSS etc. Worse thing that could happen is the machine not booting up. It has nothing to do with IMC or NB.

One major difference btw AMD and Intel is AMD goes for more pins than necessary to support a few generations while Intel goes for the bare minimum for cost reduction at the expense of more frequent socket change. But I believe given the cost of substrate and assembly going up AMD will eventually have to go for the minimum pin-count approach.

Some of the reasons:
1. Can sell more chips (most obvious) -> new CPU + new NB + new SB
There's a lot of resistance in moving from a 3-chip solution to 2-chip as you make a lot less.

2. Lowers cost
Designing just to meet current needs to lower cost.

3. Screw-ups in design/planning
Short-sightedness in planning, poor execution etc...

Point #2 is more applicable to substrate design than silicon design.

Silicon technology has evolved at a lightning pace (180, 130, 90, 65, 45 in ~ 5 years). With increase transistor shrinkage + larger wafer sizes the cost of manufacturing silicon has gone down if you don't factor in the large initial investment in fabs and equipment.

But IC packaging substrate technology hasn't kept up and it's becoming a larger cost constributor vs silicon. So the substrate is designed just to meet current needs to reduce costs. Intel could have created a socket with 1000+ pins instead of 775 and support it over multiple processor generations but it doesn't make sense from a cost standpoint.

A car is a terrible analogy. If the car industry evolved as fast as the tech industry we all be driving last year's Ferrari right now for the low low price of a Kancil.