So logicaly if i understand this corectly we should be able to bypass this rule by drinking Potion of Speed using our Bonus Action instead of casting with it.
If we go by the rules, then no: haste doesn't give the character another main action to spend on anything they want. Also if we go by the rules drinking potion should be a bonus action (that change I don't think is an issue)
Haste
Choose a willing creature that you can see within range. Until the spell ends, the target's speed is doubled, it gains a +2 bonus to AC, it has advantage on Dexterity Saving Throws, and it gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon Attack only), Dash, Disengage, Hide, or Use an Object action.
When the spell ends, the target can't move or take Actions until after its next turn, as a wave of lethargy sweeps over it.
Chaining powerful spells can become a massive balance problem in later levels (that's why part of community is so desperate to get to play with level5 characters - as this is where multiattack becomes a thing. I am not looking forward to having enemy wizards chaining fireballs at my team - and then warriors pushing off cliffs any remagning survivors.
