I think the notion of a blanket 'steam ban' on the main line is extremely unlikely in the medium, even the medium-long term. It's worth looking at Network Rail management's approach to steam on the main line in relation to that of its predecessor(s) in British Rail. When you do this, you see that the steam railtour market has expanded significantly over the last decade and a half, and steam locomotives are a relatively common sight on our railways - even hauling the Royal Train on selected occasions.
The lengths to which railway management will go to arrange and accommodate steam specials now is extremely promising - look at the Underground 150 celebrations in January of this year, which saw a 19th century coal-burning steam locomotive haul a train of wooden bodied turn-of-the-century (and older) carriages through one of the most densely operated sections of railway in the country. And to show that this wasn't a simply an exceptional one-off, a repeat is planned (but this time from Hammersmith to Moorgate) for next year, in celebration of that line's anniversary.
That said, I do think that - assuming speeds and density of passenger operations on main lines increase in line with what might be expected - we shall see steam limited more to secondary routes in the future, simply because there may not be the space available to fit these special trains into the timetable.