Rollox yourself, if you read the i-size thing properly it says that they won’t be outlawed until the vast majority of cars have the facility for Isofix which is several years away at least. Same as when they brought in mandatory seatbelt usage.

Will manufacturers stop making car seats that fit only with a seat belt?

In time yes, but this is likely to be some years away. Modern cars are designed and built to last a long time and it is is very likely that, for example, in 15 years’ time we will still have vehicles on the roads that will not have i-Size fixing points.

My understanding of isofix is that it is not necessarily safer than a correctly-fitted seatbelt child seat, but the difference is that it is much easier to fit a conventional child seat incorrectly.

I think this is the key bit – IsoFix isn’t inherently safer, but easier to ensure it is consistently safe compared to a seatbelt based restraint

On the isofix seat we have the rigid isofix bit is designed to give under extreme loads and leave a flexible tether. The safety comes from positioning the child and the seat belt in the correct place relative to each other. They are effectively big booster seats though.

The last 2 posts are the real truth, once fitted they tend to be fitted well. Might not be easier to fit, might not be safer but less likely to be fitted badly. And yes that website does not mention an outlawing of seatbelt fitted seats rather a phase out, which is logical.

Seat belts have just a bit of give in them, so that in event of a crash (touch wood that don’t happen ) the forward momentum is slowed down not so abruptly as it would be in isofix?

Of course this argument (if accepted as having a shred of proof in it) only works with Group 0 and 1 seats. When you get to Group 2, there isn’t a seat harness anyway and the child relies on the car seatbelt.