As far as I know, none of the other top 6 teams have bought anyone (Jesus was bought in the summer). So where does this 'our rivals are strengthening' always come from?
And what will Klopp get fed up with? Himself? He's the one making the choices on who we sign so if we don't and we do fall away as the doom mongers are suggesting, he's only got himself to be fed up with.
ManU have sold Depay and Schneiderlin - and will probably spend some of that. Chelsea recalled Ake to bolster their defence- and he's had a good season. They also sold Van GInkel, Obi, Oscar and Bamford - so there might be a purchase in store too. None of the other top six managers are being as vocal as Klopp about the market - although Wenger will probably not bother as usual. Wouldn't be surprised if Spurs or City nabbed a player or two. All the teams directly below the six (Stoke, Everton, etc.) are strengthening.
But as Ecnirp stated - they're all deeper than us anyway.
As for Klopp. I think he has had a few targets that the club didn't get, like Gotze and a couple of others from the Bundesliga. They need to back him.