i dont think the big players have thought through how many people will bin the leased line when fast broadband is widely available
I think they have ;-)
Many companies pay for <4 hour fix, have redundant lines, up to and over 1000Mb/s, something DSL won't be doing for a number of years.
1). You'll be more contended - do you think if BT have 200 business on an exchange they'll have a 20,000Mb/s uplink? -I suspect it'll be 100-1000Mb in truth. - Leased line is direct and not significantly contended
2). Most BT DSL systems charge ISP's per GB to send/receive data, with a leased line the ISP's cost is fixed. The ISP will either operate a 'fair usage' policy or ensure your usage doesn't exceed his profit margin.
3). As more and more people buy DSL gear & routers etc the price drops making it cheaper.
4). Support and QoS offerings will be lower on DSL products. If you have a customer paying you £1000 a month when they ask for something you'll give it to them, at £100 a month they'll ask you for another £50 a month for that 'extra' service.
5). ISP's upstream connections for LL services will tend to match the clients' SLA, same applies to DSL.
Very much a case of you get what you pay for.
If you want to downgrade to DSL then I'd get it installed, try it for a few months before you cancel your LL.