I have no insider information, so these are just my guesses:
- Planning for Civ 8 was underway even before Civ 7 was released.
- The reception of Civ 7 will shorten the Civ 7 development cycle, unless sales numbers are better than reviews and player counts suggest (possible) and/or DLCs released over the next year or two are favourably received (even more possible).
- Civ 8 will be developed under a new leadership team (this was likely regardless of the commercial success of Civ 7, and pretty much certain now).
- Even if Civ 7's cycle is shortened, it will be a minimum of 6 years until Civ 8 is released (2031 at the earliest). Six years is the gap between Civ 5 and Civ 6, and that was with leadership team continuity. Game development times have been increasing across the industry. The leadership for Civ 8 will want to take the time to understand the lessons of Civ 7. They're also likely to build a new engine from scratch (Civ 5, 6 and 7 run on a core that is now getting long in the tooth). That will take time.
- There will be no quick pivot, but there may be a diversion of resources from longer-cycle Civ 7 DLCs to core Civ 8. That could happen as soon as this year, but may not happen until the first-cycle DLC reception is confirmed (i.e. no decision until next year).
- If Civ 7 sales numbers are really bad, there may be an interim project between now and Civ 8, such as a re-skin of an earlier version of Civ, something to replace future Civ 7 DLCs and generate current revenues while Civ 8 progresses. Different parts of the development team are busy at different times in the development cycle, so efficient use of resources mean enough projects in parallel to keep all of them gainfully employed at all times.
Bottom-line: with a commercially successful Civ 7, the development cycle would likely have seen Civ 8 brought to market in about 10 years (approx. 2035). If Civ 8 is accelerated, it's still likely to be at least 6 years before it would come to market. In the meantime, we'll continue to see Civ 7 DLCs unless they really flop, in which case we may see a modest interim project between now and Civ 8.