No, it's not all about goals scored, but if none of your strikers can score more than a Norwich striker, a[n Everton] full-back and a Stoke playmaker, then that no doubt reduces your chances of winning/challenging for the league. I'm not saying that's necessarily the target, I don't know, but I'm just saying that with a good, new striker, you could be unstoppable. As you've said, you've got a fantastic defensive base to build on, with a decent midfield (maybe missing a deep-lying centre-midfielder), so a top class striker would complement that nicely. Good sides may be built from the back, but top sides are very strong in both defence and attack.
Lampard has played attacking-midfield in the past, so he is competent there. Maybe he isn't as effective as he was before, but I'm naming players who can do a job in that position in a crisis (which is what depth is all about). Bertrand can play as a defensive-winger (replacing an attacking-midfielder if needed), and although the other two won't get near Hazard/Oscar/Willian, they are worth keeping. The thing is, if you look at other top clubs, none of them have two very good players in each position, spending to get that much depth is a waste. Depth is always good, but too much depth either results in a waste of money, or a lack of playing time for some top players - sometimes, the best squads don't have all the best players, but some great players and some willing, competent and cheap backups.
And we are building for the future and next season will have a potential world class striker back at our club won't we? For Chelsea this season its not about this season, this season is to prepare us for the future.