My point was that it's hard to get tail emission right.
You
can't get tail emission right. A good TE value in a certain market conditions is an awful TE in different market conditions. The success of tail emission depends entirely on whether the number of miners stay almost-constant thoughout the lifetime on the coin. Just like in economics, there is a break-even point in the number of miners where more miners joining the network will not see an increased reward - as the price (which is dependent on users buying and selling) works independently from miners.
By supplying Tail Emission you constrain the network to a certain number of miners, and since there is no noticeable increase in hashrate, there is no corresponding increase in coin scarcity either, hence no price increase - miners will have to support themselves on a block reward (i.e. TE value) which has a fixed USD value forever.
I will get to Monero's case. There, iirc, there's "no" block size/limit. That might affect the equation - might make it different than what we have for Bitcoin, where miners can force (by plugging out vast quantities of hashrate now and then) a cluttered mempool and bigger block rewards (I know that they have to add that hashrate back before the difficulty change/detection, I know that's overly simplified and may be wrong, but it feels possible that Bitcoin miners would do dirty tricks for more money if need be).
So you're right, tail emission size may depend on more variables than we can now think of. I don't know if it's
impossible to get it right ("never say never", you know), still, close to that indeed.