>> If the oldest node is still running and in sync then why do we require the new node? What is the difference between the old and the new latest nodes?
You do not need to upgrade every while but there are some changes that require an upgrade.
According to this site
[1], most of running bitcoin nodes are 0.18.1 & 0.19.0.1
I think this is one of the main reasons why so many nodes have been upgraded.
The duplicate input vulnerability shouldn't be forgottenyou have 27 nodes on 0.8.5 & 27 on 0.15.1
37 /Satoshi:0.8.1/ Bitcoin Core
36 /BitcoinUnlimited:1.0.3/ Bitcoin Unlimited
33 /Satoshi:0.8.6/ Bitcoin Core
33 /Satoshi:2.6.0.1/ Bitcoin Core
31 /bitcoinj:0.15.5/Bitcoin Wallet:7.36/ Bitcoin Wallet for Android
28 /Satoshi:0.15.2/ Bitcoin Core
27 /Satoshi:0.8.5/ Bitcoin Core
27 /bitcoinj:0.15.1/ other (excluded)
[1]
https://luke.dashjr.org/programs/bitcoin/files/charts/software.html