Plenty of locks are mostly just for show anyway. See Lockpicking Lawyers youtube channel.

Yes, but the average bike thief is looking for an easy steal and will avoid wasting time on a locked bike. Bikes in the Netherlands usually have 2 locks (a wheel lock and an extra chain), so thieves will usually move on from those first. A portable laser might allow a thief to steal even previously safer bikes just because it makes it quicker and silent (I'm assuming both).

Probably not. Heating steel doesn't cut it, it just forms a puddle of molten metal that will cling to the base material. For cutting to work, you need to blow it off somehow.

For plasma and oxy-fuel cutting, the hot gases coming out the nozzle acts as the mass that blows it off. Air carbon arc gouging uses regular compressed air to push the molten material away. I imagine industrial laser cuttings dealing with anything larger then a few millimetres thick would use compressed air as well.

I doubt a smaller more efficient laser is going to change that reality.

It won’t be quicker. The laser in the demonstration was cutting through 100 micrometers of steel.

Cutting through a chain is going to take much much longer. There are already quicker methods to cut or break bike chains and disable locks.

Also cutting with a laser requires very precise alignment with respect to distance. You won’t be free handing this. You’d need to mount it on a device capable of precisely adjusting the distance as you cut deeper into the metal.

Plus it’s relatively easy to engineer laser resistant materials.

It is an interesting problem since the existence of one cutter forces on offense forces all defenders to make substantial changes. I suspect the most expedient way will be to semi-criminalize carry of the devices, similar to spray paint in some places.