I think Arteta and Edu used their resources sensibly.
The ongoing unavailability of Zinchenko and Tomiyasu, coupled with Timber's serious injury, made a leftback a necessity. Kiwior did a reasonable emergency job last season, but was a sticking plaster only. The fact Partey is routinely managing issues meant a central mid was imperative too. We survived last season on the basis Rice was available for nearly every minute.
It seems clear the first player we tried to buy over the summer was Benjamin Sesko though - so Arteta was open to a striker. When that didn't materialise, he's taken the decision to continue with Havertz upfront and pivoted to Merino. If we buy a striker, it has to be the right striker, fitting the profile of play - and they rarely come cheap.
Where we did go awry in the summer was letting all of Nketiah, Nelson, Smith Rowe and Vieira leave in the same window - and replacing all four with Sterling (+Nwaneri). None were the answer, arguably all four should have departed for different reasons, but each of them has contributed over the past couple of seasons challenging. It's left us too inflexible to deal with the multitude of issues we've had.
This squad has scored 88 and 91 goals over the past two seasons. It's far too simplistic to baldly state "we should have bought a striker". As it goes, with Jesus' catastrophic decline, I'd expect our next major move to be that player, giving Arteta flexibility re: Havertz again.