[HACKERS] Challenges preventing us moving to 64 bit transaction id (XID)?

    XID wraparound seems to be quite a big concern and we introduce changes like “adding another frozen bit to each page” [http://rhaas.blogspot.com/2016/03/no-more-full-table-vacuums.html to tackle this. I am just wondering what’s the challenges preventing us from moving to 64 bit xid?  This is the previous thread I find https://www.postgresql.org/message-id/CAEYLb_UfC%2BHZ4RAP7XuoFZr%2B2_ktQmS9xqcQgE-rNf5UCqEt5A%40mail.gmail.com, the only answer there is:

“

The most obvious reason for not using 64-bit xid values is that they
require more storage than 32-bit values. There is a patch floating
around that makes it safe to not forcibly safety shutdown the server
where currently it is necessary, but it doesn't work by making xids
64-bit.

"

   

    I am personally not quite convinced that is the main reason, since I feel for database hitting this issue, the schema is mostly non-trivial and doesn’t matter so much with 8 more bytes. Could some postgres experts share more insights about the challenges?

Thanks

Tianzhou