I'm sure there is some flaw in my ointment,but you conveniently avoided the question in your flip answer: What happens when you two block the shitteree at the top? I personally would be looking at that extra 45 feet of 2.00 line sittng in a heap at my feet.
Sorry, I wasn't trying to be flip in my answer.
Approach the problem this way: the spar supports the entire weight of the mainsail, there's no way around that. The adjustable loading is in how much tension is on the length of the halyard that runs from the masthead to the deck (the fall line). On a 1:1 halyard the fall line has the same load as the mainsail and the mast sees 2x the mainsail load at the maststep as compression.
On a 2:1 halyard the fall line carries 50% of the mainsail load and the mast sees 1.5x the mainsail load (1x from the sail, .5x from the fall line). The other 50% of the mainsail load is captured in the 2:1 block and tackle at the masthead and does not apply compression down the length of the spar. There is compression between the dead end of the halyard (presumably to a U-bolt at the masthead) and the mast sheave, and this compression is captured at the top of the mast and does not contribute to compression running the length of the spar to the mast step.
Even if you two-block the headboard block, there will still be a turn of halyard from the U-bolt fixing at the masthead, down to the bottom edge of the headboard block sheave, and back up to the mast sheave.
Theoretically, if you could apply sufficient tension that the mainsail halyard went bar-tight and the headboad block was now riding fore-aft along the bit of halyard running between the U-bolt and the mast sheave, at that point you have lost the mechanical advantage of the headboard block and the fall line again sees 100% of the mainsail load (or more, depending on how much tension you had to apply to the fall line to get the halyard to go bar-tight). If the mainsail is cut so tall that the luff really does go 100% of the way to the masthead, then there is no clearance for 2:1 halyard block will not fit and is of no value.
Does that make sense? It is somewhat counter-intuitive, I find it easiest to draw out on paper.
As you point out, hopefully the mast designers made the spar strong enough to handle the compression load applied by the halyards and sails. On a practical matter, I replaced the 1:1 mainsail halyard setup on my boat with a 2:1 to make it easier for me to hoist and reef/unreef the sail - 450 square foot main on a 60' spar. Reducing load in the halyard leading to the Antal clutch meant I could use a smaller clutch and still stay well below the clutch's breaking load.
For the race boats (Volvo 70s, for example) using masthead halyard locks means the spar designer could reduce spar compression load by 50% (and therefore make the spars much lighter or thinner), which is huge when you figure the load includes loads from the code zeros, giant mains, and the equally loads jibs. For just about everybody else, including me, that is not sailing a custom super-lightweight mast, the practical benefit of a 2:1 mainsail halyard is to make it easier to raise and adjust the mainsail under load.