The starters are important because you need to fill starter's innings. 5 innings a start is about 800 innings coming from your rotation. Push that to 6 and you need almost 1000 innings from your starters. You need 50 to 60 innings from your closer.
Sure, but #1 the Sox already had a great bullpen, so no problem there. The Nats comparison falls apart right there. The Sox also have a staff loaded with potential closer arms such as Garrett Crochet, Michael Kopech, Dylan Cease, Codi Heuer, Tyler Johnson, Aaron Bummer and company. I feel REALLY good about them being able to find a high level closer among that group if they needed to. Sure Hendriks is a great add to this team, but his addition is a marginal improvement over what they had last year. A higher level starter gives a LOT more chances for improvement, especially when compared to what there end rotation starters looked like last year.
The Sox could have flipped the signings upside down and signed a $13 million starter, another 3 million dollar arm in the pen, and then picked a closer from the winner of the spring competition for the closers role.