If you are a Dex based fighter/ranger/monk, you would get nothing out of the elixier apart from maybe a higher carrying capacity and the bladelock can use their charisma stat right when they get their subclass, so again, why would they need a strength elixier? You don't play half of the game with the wrong stats and then change your playstyle completely.
That bladelock is going to build 8 strength, then drink the potion and have 21 strength, and use their strength stat for all attacking and damage rolls because it's higher than their charisma, which was at best only 17.
They aren't playing with the wrong stats, they're playing with extra stats. They don't WANT to use their charisma, it's not high enough in face of the potion.. They'll still build charisma, for their spells, but they'll use strength for their melee.
It's not a comparison between a bladelock who built strength and one who didn't, it's a comparison between two bladelocks who made strength a dump stat, but one has 21 anyway, and that one will outperform the other.
Rather than progressing their melee effectiveness by raising their Cha at levels 4 & 8, they'll be level 1 with level 8 effectiveness using strength, and neglect to use their charisma until it's already 20.
Same for the monk and ranger with dex. Anyone doing melee can use str. It has no impact on their playstyle, only what stat is used for the rolls while using the same playstyle.
Likewise you'll have barbarians that dump strength, build up dex and con instead, then have 21 strength anyway.
All that extra unarmored defense in return for one little potion per long rest? Time to trivialize tactical difficulty
