Gameplay is in every imaginable way the most fundamental element of any game.
Immersion isn’t even close to mastering as much. (Though it does matter)
And sure, okay, only heavy ship require timber. How does requiring lumber for heavy ship make building light ship without lumber less immersion breaking? Aren’t you still building wood objects out of iron?
- And to get back to an earlier point: there is very little evidence for Lack of a Resource being permanently crippling to anybody until the quantities become Industrial and an order of magnitude greater than they were for most of the Game Time (like, the last 300 out of the 6000 or so years in the normal Civ time-frame).
To take Timber (meaning the heavy Ship Timbers in discussion), there was none available in ancient and classical Egypt, yet classical (Ptolemaic) Egypt built a large fleet of Polyreme warships (4, 5, 6, 10 banks of oars each) and the largest polyreme ever built - the Prestige-Only 40-banked catmaran that could carry 4000 troops at once!
- Because they had the Gold to pay well for people to bring them the Timbers - from Lebanon, Macedon, Greece, and even possibly from the Caucasus down the Black Sea - virtually everywhere with trees within commercial reach.
In addition, there are almost always Substitutes: the first coastal boats known (Egypt and Mesopotamia) were not made of Timber at all: they were of reed bundles, in the case of the ships venturing out from lower Mesopotamia strengthened and waterproofed with Bitumin from the oily pools already discovered in the region. These galleys were sufficient to carry the first troops along the coast, and shipping (including massive stone blocks) up and down the rivers.
Nor is this limited to the Ancient World: Major Combatants like Britain, USA, and the USSR in World War Two all built aircraft all or partly out of wood and/or canvas instead of the specialized aluminum alloys normally used - Just In Case they ran out of the Aluminum. Nor were these 'second-rate' aircraft: they included the British Mosquito bomber/night fighter and Pathfinder, the American F4U Corsair fighter, and the Soviet La-5FN air superiority fighter, all considered among the best of their types.
Lack of resources is limiting mainly in that it makes it more Expensive to build something, or requires more work to find a substitute - perhaps even substituting a new manufactured or traded Resource, substituting Gold or Trade for Mining or Refining to get what you want or need - but it doesn't stop you from doing something that your Civ needs to do.