For the purposes of environmental hazard, it isn't number of transactions that matter but number of miners. That in turn is going to depend on the expected payout to miners (either in value of new coin or in transaction fees). New coin mining rate is fixed for bitcoin, so the only parameters one can change to reduce the environmental impact are value and transaction fees.

If you're looking at in in terms of transaction on the blockchain perhaps, but the reality is that a single transaction on the blockchain actually maps to many other transactions (for example, exchanges batch their outputs, so even though it appears as a single transaction, they could be paying hundreds of people at a time, likewise for lightning or other layers).

But as the coin with the largest market cap, greater than the next 9 put together [1], is by far the biggest one.

I agree though. Bitcoin isn't the only crypto coin worth talking about.

[1] https://www.forbes.com/advisor/investing/top-10-cryptocurren...

If transactional demand were the only useful adoption metric for money, where would that leave gold?

The most important factor in a particular asset’s “adoption” as currency is its propensity to act as the unit of account for international settlement between nation-states.

The absolute number of transactions is of little importance. Bitcoin moves around a billion+ dollars an hour. Ethereum is 1/10 of that. So Bitcoin is definitely #1 in terms of transacted value per day, which is the only important measure of adoption.