The 2 rival operators are taking up routes under contract to Slough Borough Council, First are giving up these routes which they currently operate on a commercial basis, this is now a universal practice across the UK. (A cynic might say it follows a predictable path, Operator announces cuts/Service withdrawals, usually, (but not always) The 'Public' protests via local media, sometimes local councillors or MPs make a noise, the local council makes some noises and, depending on funding and local politics, puts out tenders to provide a contracted replacement service).
I would think that the days of First acting 'strategically' to keep competitors out of an area/geographically defined market are long gone, First need to wring the last penny of profit out of their operations.
First Bus March 2017 financial results, show most off their subsidiaries making an operating profit and then in most cases making a hefty contribution to the DB pension scheme, a sensible strategy although possibly one that they may not in reality have had much choice in.