Item levels interaction in PF2 is for only two reasons:
-Give a rough idea of what items are appropriate but there are no item level limits, a character can perfectly find a level 10 item at level 1, if a gm feels so inclined.
-Crafting, the game pulls a full stop on making items of higher level than you. Now you must be at least higher or equal to make item level x.
Items with no listed levels, are considered level 0 items, which makes it easier to craft. Besides that, no scaling up like starfinder.
The item level mecanic is clearly taken from Starfinder, and P2E uses it. Many items have levels, but it's not the case with regular mundane weapons.
Was it intended to be that way, or was it intended to add upgraded versions later on, such as say, a longsword dealing 1d12 S with a 10 item level?
Page 271: "If an item’s level isn’t listed, its level is 0."
There is no such thing as a 1d12 longsword, but there are magical weapons such as a +1 striking longsword, which is higher level.
was it intended to add upgraded versions later on, such as say, a longsword dealing 1d12 S with a 10 item level?
Also, that is severely underpowered for a Starfinder style level 10 weapon. Higher tier weapons would do additional dice worth of damage, not larger die size damage.
Which is exactly what the Fundamental Weapon Runes add to the weapon.
So the Starfinder weapon item levels found their home in PF2 in the weapon runes, rather than the weapons themselves.
I think the big difference is thus- in Starfinder, you can build a plasma cannon that is much more effective than a standard plasma cannon using just technology. Pathfinder folks don't have a technological way to make a sword much more effective than a standard version of that sword- if we want a much more effective sword we have to use magic runes.
Which is why the levels are on the magic runes, not the items.