After the emotional buy-in of the first 6 verses of the Old Testament reading (see yesterday's post), the last verse hits hard with the meaning. The vineyard owner is God, and the worthless vines are his people - the people Isaiah was telling the story to. This is an allegory about the audience and it's meant to hurt, to get their attention.
Back when the Soviet Union was a thing (and Yakov Smirnoff had a career), the tastefully (?) embalmed body of Lenin was put on public display in Moscow's Red Square. It's still there and if you see someone looking at it today, chances are they're thinking "Is that thing real?" With the fall of the USSR, the impact that the body of the founder of Russian Communism was meant to have was lost.
By Jesus' time, the Old Testament reading had also been tastefully embalmed. Instead of being read as a warning or a prophecy, Isaiah's pointed allegory was being read in all kinds of ways (Adam and Eve's fall from grace, various foreign invaders, etc.) to make it apply to...just about anyone but the people hearing it. The impact that Isaiah's allegory was meant to have was lost.
Which is why (in the Gospel) Jesus re-tells it with a twist that gives it back its meaning and then some. More on this tomorrow.