Bigpipe is owned by Spark. I feel this is different, like MyRepublic are staying but they don't want to do NZ internet anymore.

I think Spark recognised the value of having different brands to hit different markets, but they still get the benefits of economy of scale. E.g. routers all shipped from same warehouse but on different box depending on ISP brand, one call center that tells staff what ISP you're calling to give the correct greeting, etc. They probably also think it's funny when people get angry about something and run to a different ISP without realising it's also Spark.

but I paid a one-off fee for a static IP some time ago and would be a real pain to lose it, especially when most providers these days seem to charge a monthly fee for a static IP rather than a one off cost (presumably because of the shift to CG-NAT)

They charge the monthly fee because people pay it. CG-NAT isn't even that common, and IPs generally don't change unless you restart your router. But these days if you're ok with cloudflare then it's pretty common to use Cloudflare Tunnels, which make an outbound connection to cloudflare to remove the need for port forwarding and a public IP.