For very early stage startups, I think outsourcing software development can sometimes have merit. If the main point of the software is answering the question, "Do we have a real business here?" then if there isn't a technical cofounder I think outsourcing can be worthwhile as long as you assume that all the outsourced code will be thrown away.
But as soon as the founders are sure it's a real business, I think at that point you have to toss out the oursourced stuff and start building the real code base. Then you're reasonably sure that some of the code you're writing is going to be delivering customer value over the long haul, so it's worth investing in sustainability.
There are other software companies out there, smaller and leaner than Intercom that are eating their lunch by offering similar or a better product at a lower cost. Those companies aren't 'outsourcing undifferentiated heavy lifting' but rather have a lean and talented team that's able to do the heavy lifting themselves without sacrificing on product development.
And many companies used to make outsourcing decisions based off of simple head count cost and didn't factor in the extra time/hours/delays due to integration/poor work/other factors.