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Ethereum and Bitcoin mining work like this:

Take a freshly baked pie, and give half of it away for a suggested donation of $1 USD to the first 10 people who show up.

Now split the last half of the pie into smaller chunks and slowly give out bread crumbs to new "miners" with the requirement they spend large amounts of capital on exceedingly inefficient number guessing machines in order to win the lottery of "mining" more crumbs.

  Satoshi Nakamoto
  Thu Jan 8 14:27:40 EST 2009
  I made the proof-of-work difficulty ridiculously easy to 
  start with, so for a little while in the beginning a 
  typical PC will be able to generate coins in just a few 
  hours. It'll get a lot harder when competition makes the 
  automatic adjustment drive up the difficulty.

  first 4 years: 10,500,000 coins
  next 4 years: 5,250,000 coins
  next 4 years: 2,625,000 coins
  next 4 years: 1,312,500 coins

The 10,000 BTC pizza is a case study in how Satoshi designed the supply to rapidly dump very cheaply before other users were able to discover the network, by that time and later new users not only need to spend more to mine but less coins are produced for a correct lottery number.

  In economics, the Gini coefficient is the standard measure 
  of how inequitable a society is. This is tricky to 
  determine for Bitcoin, as it's not quiet a "society" in 
  the Gini sense, one person may have multiple addresses and 
  many addresses have been used only once or a few times. 
  (The commonly-cited figure of 0.88 is based on one small 
  exchange in 2011.) However, a Citigroup analysis from 
  early 2014 notes: "47 individuals hold about 30 percent, 
  another 900 a further 20 percent, the next 10,000 about 
  25% and another million about 20%"; and distribution 
  "looks much like the distribution of wealth in North Korea 
  and makes China's and even the US' wealth distribution 
  look like that of a workers' paradise

  Dorit Ron and Adi Shamir found in a 2012 study that only 
  22% of then-existing Bitcoins were in circulation at all, 
  there were a total of 75 active users or businesses with 
  any kind of volume, one (unidentified) user owned a 
  quarter of all Bitcoins in existence, and one large owner 
  was trying to hide their pile by moving it around in 
  thousands of smaller transactions. (Shamir is one of the 
  most renowned cryptographers in the world and the "S" in 
  "RSA encryption")"

Ethereum:

  Presale ICO / Premine ( max cost $0.50 USD per ETH  )
  = 72,009,990 ETH
  
  Total Supply today (Feb 23rd 2018)
   = 97,800,000 ETH

  Source:
  https://etherscan.io/stat/supply

By your logic if that were the case, Tether would need to be destroyed everytime the crypto-markets rise...

Plus, Tether's offical website and documentation state that's not how Tether is intended, they claim it is always 1:1 backed by a real USD reserve in a bank account (which they tell people they will audit, but the auditor backed out after giving a statement that said Bitfinex would not provide the proper documentation and access, later erasing any mention of having been associated with Bitfinex.)

Monero changing its mining algorithm doesn't change how most of the supply was produced and given away nearly for free to the dev team and early users.

The math behind it equates to existing capital wealth being used to generate the supply which is amplified by several magnitude for the first users to show up and run the software. Future production costs increase, and the relative newly produced supply decreases.

Old users rely on the gullibility of new users to exchange real world value for these easy to produce database-tokens.

Why would anyone want to use a cryptocoin though a service like facebook?

Venmo / ZuckBucks / Paypal / Patron all are much simpler and more efficent for payments. Plus you don't have to buy the cryptocoins from someone who generated nearly for free and then and then jacked up the price, with Venmo etc it's instant and hassle free.

It never ceases to amaze me to see Bitcoin / Ethereum users try to come up with what-if services in an effort to trick others into thinking the database token is worth anything to other people.