700K Seems huge, but given they are only asking for interest not any membership or funding. It is dramatically less impressive then the number of 500k labour members under Corbyn,

And for the party to have a fighting chance. That level of support and funding will be needed. Just to compete with the right wing corporate funding of the current Labour party.

That said. Even if it's simply an attempt by left voters to push labour to the left. Such bodes well for the party when it dose form properly.

The biggest risk is the left wing history of rejecting parties over individual idealism. Such a party can only have any effect if the left is willing to come together over disagreements. Much as the right will to avoid the left.

If the left as a whole consider moving labour to the right important enough to unify. Either the new party will take over. Or labour will consider how dangerouse the lose of the left can be.