My post would have been more accurate had I said "a huge amount of pressure can (will) be brought to bear to move members to the new scheme" rather than "a huge amount of pressure can (will) be brought to bear to close the present scheme".
BDiONU, they can't close the present scheme under existing guarantees. Do you trust those guarantees to still be in place in 10, 20 years time?
Management have many tools to force members from the present scheme into a new scheme. Who's going to look after present scheme members interests when 60% of the workforce are in the new scheme? What about 80%? Your estimate of a 20 year 'tipping point' when new members outnumber old is a lot higher than union estimates.