the ~2.1mb POTENTIAL that segwit offers is:
only 4500tx/~2.1mb
IF 100% of users have moved funds to segwit keys. and continue to only use segwit keys (not gonna happen)
its not going to change right at activation day.
its not going to change just by using a specific node.
it will only change when people move funds to a segwit key and then use segwit keys only,
and only grows at small amounts depending on how popular the use of segwit keys are UPTO the max POTENTIAL
so lets explain it. in regards to impact on tx demand
the mempool is nearly always well above 4500tx of unconfirmed tx's(2mb) in the mempool.
https://blockchain.info/charts/mempool-size?daysAverageString=7×pan=1yearever since october the lowest the mempool has been was 3mb+(meaning 4mb DEMAND if you include the 1mb that do get confirmed)
there is also currently 47million unspent outputs that would need to be spend to move everyone over to segwit keys
http://statoshi.info/dashboard/db/unspent-transaction-output-set
so even if segwit activated lets say tonight.. nothing changes.
lets say people downloaded segwit nodes and took a few days to sync
and then lets say EVERYONE (even the holders from 8 years ago) moved their funds to segwit keys.
do you understand the congestion it would cause to get funds moved to segwit keys before full benefit of segwit can be seen
imagine it everyone trying to move funds all at the same time...
then once everyone moved across.. that >3mb+(average demand) after everything has cooled off, results in still not enough even with the 1.1mb extra "boost" segwit offers
however just increasing the blocksize to 4mb without the bait and switch will alleviate things and people dont need to move funds to special new keytypes to see the benefit.
segwit REQUIRES people to move funds to new special keypairs that are not tested on bitcoins mainnet and do offer a new attack vector by manually copy and pasting the segwit keys from a segwit node to a native node and then messing with it as a 'anyonecanspend'
yep core might filter out tx's being sent automatically by a segwit nod to native node. but they cant stop manual copy and pasting between a segwit node and native node.
in short. segwit opens up new attack vectors. doesnt guarantee 100% utility of segwit and doesnt disarm the malicious users.
p.s my over use of HR lines are for those with short attention spans who fail to read code/literature about how bitcoin works simply because 'its too long'