Yet another post pushing the same old arguments. HS2 is a very large project, and within that there are many unknowns that won't be resolved until the project is nearly completed. Compare the percentage increases in costs with recent motorway schemes:
new lane on the M5 between junctions 19 and 20 which was budgeted to cost £6m, rose to £8m in a revised estimate but cost £17m to complete
crossroads on the A14, budgeted at £5m, which was revised to £6.7m but cost £13.4m to build
see a common theme here? These are just simple laying tarmac to widen/improve traffic flow (for a couple of years until it builds up and uses the extra capacity), little or no technical risk but the main difference has been no persistent attempts to delay the programme which
always increases total costs. Crossrail suffered whinges for most of it's planning and building years with predictions that it wouldn't be needed/was technically too difficult to build/was costing too much 'taxpayers' money*/etc., but with it's success since fully opening, there's a distinct silence from the "told you so" brigade. As
@Wolfie says in post #18, so much of Crossrail's cost came from London itself so doing something to what has now become an essential part of the Capital's strategic transport network will be quickly stamped on.
I can't see anybody that matters changing their mind on completing the build as currently defined including Euston. Trying to rearrange two existing tube lines, two branches of the Overground and trashing the utility of Crossrail that has been delivered even with post pandemic passener demand, is a non-starter!
* conveniently forgetting that it was costing London's taxpayers money which will enable London the continue to subsidise running the railways elsewhere.