Its not money laundering or lack of ambitions or anything that complicated.
The simplest reason is allways the one people are blind to. We saw an opportunity to sign a young talent for cheap (his team relegated), one who they most likely don't see as ready yet. But most importantly Sandro can't be moved, he won't move and Pellegrini seems to be staying when he was rumoured to leave couple months ago.
Ergo there is no room for a 3rd LB.
This kid would not likely be good enough solution as starter yet (planning for further ahead, if they didn't get him now Inter would have), so a bigger issue is why we couldn't done more preparing for this (getting rid of Sandro with a loss or sell Pellegrini), and went after a real upgrade in LB.
And the biggiest conundrum of all. Why the fuck did we renew Mattia De Shittio.