"Easy" is not a measure of cost. Getting rid of mold is more painstaking work, but it is less resource-intensive than rebuilding.
As another example, amputating a limb is relatively easy as surgery goes, but it will cost you your limb and you would probably prefer that doctors not do it for their own convenience.
"Easy" only fails to be a measure of cost if you don't account for all the people involved.
The sum total of amputation is much harder than fixing the limb, assuming it's possible. Yes, it's easy for the doctors, but it's hard for you.
If fixing mold in a house is cheaper than building a new house, that's telling you that it's easier overall. It might look harder, but that's because much of the difficulty of building a new house happens off-site, in the felling of trees, mining of gypsum, fabrication of pipes and wires, etc.
If Tesla is doing warranty replacements instead of repairs it's not because it's "easier" in the sense that the technicians do less work at the expense of the company, it's because it's "easier" in the sense that it's less effort and money in total.
It seems not entirely unlikely to me that either Tesla is temporarily choosing the more expensive option because they don't feel they have good handle on the problem (e.g. they aren't sure their repairs will stick), or they're actually doing both replacements and repairs (with your replacement being somebody else's refurbished model).
I'd put my money on the latter, although they could pretty easily transition back and forth between the two as they needed. Start with new replacements, check out the old ones, see what went wrong, fix them up if you can, then use them for new service. If you have a lot of nominally identical items out there, why make the customer wait for a repair when you can swap in a new/refurbished unit, then repair their old one at leisure?
