There is always a lot of packaged up implied assumptions in stuff like this.

Maybe there are problem domains where immediately rewriting everything in Scala and applying TDD would be a huge win over some PHP that "seems to work".

Of course you have to weigh short term vs long term consequences and not rush to conclusions (especially if you are new). You may have hiring difficulties but that may work in your favour (see "python paradox").

Also I think that the best way to write code is to assume that bad code will be written and apply stuff like unit testing as a means to make replacing that bad code as easy as possible.