>But the limit in the number of transactions is not a technical limit, it's a political limit. We chose arbitrarily to set the limit to 4 million segwit weight units, however it could be changed to anything without impacting energy consumption.
It's at least partly a technical limit, because there are technical issues faced by scaling the Bitcoin network to the arbitrarily large number of transactions/sec you think it can handle. Each scaling solution has its tradeoffs[1][3], and therefore, to the extent that there's no "free [scaling] lunch", then there is inherent scarcity to what the miners can support, and therefore transactions should be attributed their proportional share of that limit when accounting for them.
It only appears as a political limit because a) users generally don't want a fragmented network, and b) there's persistent disagreement on what tradeoffs are acceptable when scaling.
>Therefore your analogy is incorrect: if the cargo capacity of a truck is increased, it will certainly impact its fuel consumption.
What is that responding to? The issue was how to account for the cost of one unit that takes up a scarce slot in a known-cost thing. The fair (IMHO) solution, per previous comment, is "blame" each unit for its share of the total cost.
How does an increase in the truck's capacity refute the analogy? The same logic would hold: if trucks could carry more Netflix DVDs at the same [environmental or whatever] cost, it would be reasonable to say that the cost per DVD shipped also went down. If trucks had infinite capacity, then it would be fair to say that the DVDs have no cost, since the cost inputs are independent of problem size (number of DVDs to ship). But -- per [1] and above -- we don't live in that world.
[1] For example, if you simply said "each block can hold as many transactions as miners are willing to tolerate" [2], that risks making the ledger (blockchain) freakishly large. Since each node has to store the full ledger, that blowup, in turn, risks [excessive] centralization, where only a few miner-datacenters can run nodes, defeating the (or a major) point of Bitcoin.
[2] which you think would make transaction capacity non-scarce
[3] Wiki page for discussing the issues: https://en.bitcoin.it/wiki/Scalability