I don't think that implication is there at all. It absolutely doesn't hold for strong notions of equality - strong equality of all languages should imply that they all share a single set of tradeoffs.
It also clearly doesn't hold for "equal suitability to any purpose", since something less suitable to the purpose may be traded off for something more suitable.
I think any general notion of "equality of quality" can probably be viewed as a special case of the above reasoning - we are picking a set of attributes, of benefits and drawbacks - but we've no guarantee that they all net out the same however we are measuring them.
PHP isn't a bad language if its tradeoffs work for you. But you would be hard pressed to see anything good today in BASIC from the 80s (like we might miss things if we all of a sudden were sent 1000 years back in time). There is no best PL, but there are new PL technologies that make programmer experiences better; using a language that doesn't include at least a few of those technologies is probably going to be a handicap (compared to your peers who are using them). Haskell is bit up the tech tree, but not incredibly so compared to other modern languages like Scala and even C#.
To say "every language design has its own set of tradeoffs" implies that all languages are equal, but that's clearly not true; we can all think of languages that are worse than others. I'm not at all convinced that there isn't an overall best language out there (I don't think it's Haskell, FWIW).