If this was recognized then as a border, it would have been so much more beneficial
Well, Chrobry wasn't clairvoyant, he couldn't tell upfront which territorial gains will "stick" over time and which won't. Today's borders include his Pomeranian gains, but not his Ruthenian ones. He couldn't have known that back in 1018. You could say a similar thing about pretty much any country in Europe that expanded at some point. Lithuania once owned Ruthenian lands, today Lithuania is back to its small, ethic self. You could say that Lithuania could've "skipped the whole GDL" and save time. Sweden once owned Finland and Estonia, today Sweden is back in its (largely) ethnic borders. You could say that Sweden could've "skipped the whole Finnish adventure" and saved time. Denmark once owned Norway, today Denmark is back to its small ethnic self (plus Greenland, see PS below). Germany made big inroads into East-Central Europe (East Prussia, Silesia, Pomerania), today it's back where it was ca. 1000 (plus the land of the Polabian Slavs, who never managed to wholly unify and form an independent state). Today's borders won't change the sense of achievement borne out of those historical expansions. Also, even though e.g. today Lwow/Lviv is outside Poland, it won't change the lasting, historical contribution to the Polish (sometimes world) culture and science made by Lvovian Poles while Lwow was Polish. Born there: Stanislaw Lem (Solaris), Stanislaw Ulam (hydrogen bomb with E. Teller), Maria Konopnicka (Rota), Alexander Fredro (Zemsta), Stefan Banach (Lwow School of Mathematics), etc.
PS. The territorial gains in or around Europe which seem to "stick" over time are large, uninhabited areas, like Russia's Siberia and Denmark's Greenland.