Ironically, a defined contribution (money-purchase or DC) vs a defined benefit scheme (final salary or DB) are fairly similar in costs to the company. It is perhaps the certainty to the company of payments under DC, combined with the introduction of accounting standard FRS17 that have prompted the move from DB schemes rather than the long-run cost.
For DB, since the benefits to be paid are dependent on future events, such as the average or final pay of the employee and the remaining lifetime of pensioner members of the scheme. The impossibility of predicting perfectly the pensions, benefits and investment returns on contributions means that the pension cost ultimately attributable to each financial year can only be fully determined when the scheme is being wound up. In the meantime, it has to be assessed by actuarial calculation (every 3 years).
So move to DC and reduce actuarial costs and move risk to the employees. Further, FRS 17 makes radical changes in respect of accounting for defined benefit schemes, leading to increased volatility in the balance sheet as actuarial gains and losses are recognised immediately and scheme assets are valued at fair values. Previously when the pension's investments moved about, the gains and losses (being paper losses or profits until the investments are sold) were not recognised like this, and the scheme was left to recover in the next rise in the markets.
Basically the accounting industry wanted transparency on the unrealised losses, but have ended up screwing us lot who are yet to enter our aviation job for life, simply because market downturns will eliminate profits and gains on a company's P&L or STRGL respectively, as the 5-years for which the gains or losses are recognised may be too short in longer downturns such as now.
The new and old accounting rules may produce different, and possibly very different, results. Such differences will arise because the bases used are different, and also in some cases because the valuations will have been carried out at different dates.
Unfortunately it will now mean that dependent upon the point at which we retire, our benefits may be better than under a DB, or worse, wherever the market and annuity rates stand.
That is an accounting viewpoint - I cannot tell from a pensions industry viewpoint any further reasons for the changes unfortunately!