I share your concern and it's a fair call but I'll counter with this.
Our defence wasn't the only reason we lost so many games last year. We never got going last year really. The Everton and Chelsea games were the only ones where we looked like Poch's Tottenham. I think that was down to midfield where Dembele was crocked, Dier was crocked, Winks was crocked and he had to build his midfield around Sissoko. We didn't control games as a result. Next season should surely be different. Hopefully Dier and Winks stay fit, Ndombele is here and Lo Celso can play there too apparently.
Jan and Toby being a year older is a concern. But Sanchez is also a year older and he will become a beast. Hopefully he's a few steps closer to it now.
The full backs are the main worry and, for me, RB. I think Rose and Davies are great options at LB. However, Aurier, Foyth and KWP aren't. I'd love Aurier to become the player I really think is in there but he's had two years and not done it. Foyth is a very good stop gap but not the sort of full back a Poch system needs so that position is my biggest worry.
However, if we go into a season with Kane, Son, Moura, Lamela, Lo Celso and Dybala as our attacking options with Winks, Ndombele, Dier and Sissoko behind them...fudge me I won't sleep tonight just thinking about that.