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Isn't Bitcoin Unlimited allow the blockzsize to be set by the market without the need for a tail emission?
No.
Bitcoin Unlimited relies on the cost of orphan blocks to create a fee market for the blocksize. This is based on the work of Peter R. Rizun
https://www.bitcoinunlimited.info/resources/feemarket.pdf. The key conclusion in Peter R. Rizun's work is that the cost of increasing the blocksize is proportional to the block reward.
R. The trouble with this is that if
R = 0 or becomes very small then there is no cost to increasing the blocksize, and no incentive for miners to secure the coin. It is exactly the same problem as using the Cryptonote / Monero adaptive blocksize limit without a tail emission, as is for example the case in Bytecoin. The penalty is also proportional to
R so without a tail emission there is no cost to increasing the blocksize and no incentive for the miners to secure the coin. The Orphan Block market is of course also present in Monero in addition to the adaptive blocksize limit.
The advantage of the Monero approach, in addition to the tail emission, is that there is a fee market to control the blocksize well before the blockchain is taken to the orphan block limit. The Bitcoin inflation rate will fall below that of Monero withing 12 years, so tail emission is not a far in the future issue. Bytecoin may also provide an early indication of this failure ironically because the Bytecoin ninja / pre-mine accelerated the Bytecoin emission curve by two years. This is why I prefer to think of Bytecoin as the "canary in the coal mine" as opposed to the "scam in the ninja / pre-mine".
Edit: The question of how will the POW be secured after the block reward runs out was never answered in the original Bitcoin design, and this omission lies at the heart of the "blocksize" issue in Bitcoin. This omission is also present in most POW coins with Monero and Dogecoin as significant exceptions.