Technically, Blockchain is an inefficient (bad) data structure that is supposed to control transfer of assets (an important infrastructure). So yes, it is objectively bad just like O(n) is objectively better than O(e^n).
"Blockchain" is basically just a Merkle tree with a branching factor extremely close to 1. That is, in and of itself, neithre inefficient nor bad.
However, pairing that with a proof-of-work consensus mechanism is incredibly inefficient, energy-consuming (requires at least 50% of all available compute to be engaged with the blockchain or risk bad actors taking over).
AFAIK There is no application of Blockchain other than consensus. Do you know of any to justify this distinction? (also why merkle tree? looks like a plain old list to me, with very few exceptions while it is being distributed)
Well, a degenerate Merkle tree is a list (any degenerate tree is a list). As a consensus mechanism, I could potentially think of many ways of getting that, but none that are both efficient and trustless.
As-is, the cryptocurrencies we've seen have all been incredibly wasteful and inefficient. Any that would be efficient, both energy-wise and for speed-of-transactions would most probably require ditching complete decentralisation and thus be unacceptable to hard-core ideologues in the "crypto space".
For a rough estimate of the "transactions per second" you'd need to be a global, practical, "we handle all transactions", we can do a rough estimate. There are, what, 6 * 10^9 people on the planet. At a conservative guess, each person does about one exconomic transaction per day. Let's assume that is approximately evenly distributed through each day. A day is 86400 seconds, that's close enough to 10^5 that we might as well use that. That gives us at a minimum 6k transactions per second to be viable for a single global electronic currency.
However, pairing that with a proof-of-work consensus mechanism is incredibly inefficient, energy-consuming (requires at least 50% of all available compute to be engaged with the blockchain or risk bad actors taking over).