I've just been thinking about what has happened to the former Sovereign operations in Stevenage since Arriva took over in 2004
44 - Contract lost to another operator
300/1 - Split into 300/1/2, about to be changed again losing the 300
315 - lost to another operator
383/4 - lost to another operator
SB1 - Now just the 1
797 - withdrawn
Just leaves the 301 as the only service with the same number. It's not to say that routes wouldn't have been lost or withdrawn if Sovereign had remained but it's quite a noticeable drop in operations.
The SB services are a strange one, once Arriva took over all the Stevenage local services they renumbered all there's from 2 to SB2 etc only to reverse it a few years later back to single numbers, however they now have one Streetlite in heritage SB livery. The SB10 and SB15 are Herts contract services which always seems to avoided the bigger operators still retain their SB prefixes.
A slightly partial view of the world that though.
On the 300 / 301 - they've maintained a basic pattern of 2 buses / hour between Welwyn GC & Stevenage, which if you go back to the 1980s was the 300 / 303, there are 4 buses an hour between Welwyn GC and St Albans - which again was broadly the case from the mid 80s and 2 per hour between St Albans and Hemel, so there's not exactly been a huge retrenchment there.
The 44 has been an HCC contract for years and has changed hands several times - I think Centrebus have had that a while. Didn't Sovereign only pick that up when Semarks / Dunn Line collapsed ?
The 315 is similar and is only a Mon-Fri operation, again I thought Centrebus had held that for quite a while. I know Reg's had it at one time.
The 383/384 again had changed hands several times - at one point since deregulation Luton & District ran it.
And the 797 was the last of the Green Line London services from North or Mid Herts - and Uno tried to keep that one going with no joy, so not really fair to blame Arriva for not trying on that one. in fact at the original split of the Sovereign Stevenage operations, the 797 went to Luton & District and then returned to Sovereign. The Green Line network into London has been in freefall decline since the early 80s mainly because the rail service has continually improved.
Of those two SB routes which are contract services - I think they've been contracts for many, many years - ISTR Chambers Coaches operating the SB10 at one time in the 80s or 90s.
The vast majority of those you mention were contracts which had frequently changed hands, the basic Stevenage - St Albans - Hemel service pattern is still in place and the SB1 still runs as well.