Replaceable?

Everything in life is unique and to a degree be substituted for...
Never exact, and never the same
Nonetheless, certainly replaced
...
Some replacements will be upgrades
While others, downgrades
Satisfaction may soar or may wane

This I know for fact...just when I think...I would swear to know
That someone is irreplaceable...
Not only do I find a replacement...I then wonder,
"What the heck took me so long to change it/this/them"
"My God"
Long over due!!!