If I want to spend 0.4BTC and I only have a UTXO for 1.0BTC, I will send 0.4BTC to the seller's address and 0.6BTC to my address. The seller has one UTXO and I have the other.
One UTXO has become two.
As time passes and more transactions occur, the number of UTXOs increases monotonically and the value of each will fall monotonically. (Right?)
If we extrapolate, the value of the individual UTXOs will tend to 1 Satoshi.
Or to ask the question differently, can two UTXOs ever be combined into one UTXO?