By no means am I saying that it is an equally-easy task on all makes - of course every phone will have a varied degree of difficulty when it comes to flashing, but don't you think that's a matter of interest for the users of particular phones? I'm also not saying that all Custom ROM's are glorious, I just said that the great majority is actually an improvement over the stock ones. I don't think that the stock firmware is always 100% stable - that's not a general rule, that's a matter of who designed and customized it - there are better and worse stocks.
I also disagree with your point that phones are upgraded because of the necessity of communication - communication implies that a phone is supposed to make phonecalls and send texts, everything over that is extra functionality. You don't even need a smartphone to satisfy that need and you don't need any updates to facilitate it - that much I can assure you of.
When it comes to development, it is only "quicker" because many smartphones are really based off of similar blueprints with certain parts replaced with different ones. Not once and not twice have I seen two smartphones of basically the exact same make but marketed as two different products due to some superficial difference.
Not only that, not all people buy phones on a contract - I actually buy phones boxed and SIM Unlocked, simply because I like my number enough not to change it and I never get "neat enough" deals to actually decide on one.
Other then that, I agree with you, however I do believe you also see my point, even though I grasped it quite crudely.