In most cases I wouldn't suggest this. But that would level out the threshold, and if it's straight on concrete it shouldn't deflect any or crack much. It's actually a decent use case. Basically using the old tile as an extension substrate.

Always looks crap, never settles well, transitions look silly. Worse, you're just leaving double trouble for the next person down the line. Do the job properly (I just spent a month removing a double tiled floor).

I bought a house built by the person who built the terrace of houses of which it's part, he built this place for himself. One look at the roof (no lead in the rain channels, joists too thin) and I knew he'd cut every corner imaginable. However, it's huge, south facing at the rear, with a triple sized garage outbuilding which is also south facing, and I'm handy.

Paid for the full flight survey, got a whopping discount on the condition of the building. BUT year round solar from sunrise to sunset on two roofs means tiny electricity bills, and I got a large workshop to play in.

Man, do I curse the work of the bloke who built it on the daily. I think he built it from the spare parts he had left from the other houses, nothing about it makes sense. Bizarre drainage, floors in thermal contact with the outer skin of the building, garden walls build on six inches of micro gravel, brick interior in the chimney, the roof looks like it was built from plans made by a child.

However, if you like DIY, want free electricity, lots of space, and a big workshop you couldn't otherwise afford, then removing the artex from every inch of every wall in the house was worth the effort.

All that said, don't half arse it.