Actually it's quite avoidable. Any competent civil engineering consulting firm could have devised a scheme where the junction tunnels (there are only two) could have been built around the existing ones with minimum disruption. It's been done umpteen times in London as the network expands, and in many other cities too. The most recent in London was when the Jubilee Line extension was grafted onto the existing line just outside Green Park station. There were just a few weekend closures then.

But it would be more expensive to do and BAA have chosen the cheapest solution, eg shut the T4 loop down for nearly 2 years. Of course it's London Underground's revenue loss, T4 users will find the BAA-owned Heathrow Express more attractive now. Now guess who gets the extra revenue from that.