FormerlyHemlock
Hero
You only have one bonus action or action per turn. If you use your action to cast expeditious retreat you have a bonus action to use it but no action with which to attack. Expeditious retreat can be cast as a bonus action. If expeditious retreat is cast as a bonus action then the regular action for the attack is there but the caster no longer has a bonus action left with which to dash because it's been used. That's the loss on the casting round. It exists because of the number of allowed actions and bonus actions per turn. There is always only one bonus action per turn so it cannot be taken twice.
Expeditious retreat competes with all concentration spells because it requires concentration. Using it precludes and ends any other concentration spell. That means the 8 hr hex that would have lasted all day just cost more spell slots, but I don't consider it really a given anyway. Lost is lost. The blade warlock doesn't need to give up concentration or spell slots to cast expeditious retreat either way.
You're misreading the spell. It doesn't cost a bonus action to Dash on the round when you cast it. I'll quote the spell text again:
"when you cast this spell, and then as a bonus action on each of your turns until the spell ends, you can take the Dash action"
The bonus action cost is only on subsequent rounds. (BTW, you can't by RAW cast a bonus action spell using your regular action, the way you suggest. That's a reasonable house rule but it shouldn't be assumed. I'm sure you knew that but I'm pointing it out JFTR.)