It probably is, at first. But in an economy of different parties, who produces more needs some buyers to keep growing. If there are not enough buyers, overproduction and economical collapse will knock at the door. You can see this trend internationally: we seldom see a country be an outlier in the yearly growth. Either all countries grow, or all countries decline. It is the % that changes. We saw this during the 2008-2012 crisis in particular.
It is in the interest of the richest areas to help the poorest. By doing so not only the average quality of life of a country increases. With less economical inequalities between regions, the once poorer regions are buyers of goods of the once richer ones.
While you correctly associated Lombardia and Veneto to Germany, you forgot to mention that after the WWII, Germany went split politically and economically. When it got re-united in 1989-1991 East Germany was economically very diverse from the more developed West. Our fathers were wiser: instead of doing referendum demanding taxes to be withheld in the West, they demanded to be taxed to help the East. [source https://en.wikipedia.org/wiki/German_reunification#Cost_of_r... ] After 20 years those differences are still somewhat visible, yet they do not constitute a problem for Germany's growth and quality of life as a nation.
On the historical side it has been confirmed that 2/3 of the gold resources in Italy came from Regno delle due Sicilie [source: https://it.wikipedia.org/wiki/Regno_delle_Due_Sicilie in Patrimonio e finanza section ]. Those lands are today's most rural parts of Italy, yet contributed the most to the wealth of Italy as a nation.
I highly suggest a read on this last historical matter: Terroni by Pino Aprile. You might find a different point of view. [ https://www.amazon.com/Terroni-Ensure-Italians-Became-Southe... ]