your scenario is pessimistic but realistic. Potong Pasir and Hougang in oppositions control are vulnerable once their long serving incumbent MP leave. 人在情在 人走情走 - Chiam and LTK iron votes might not go to their successors. If that happen, PAP has excellent chance of winning back the seats
anyway, putting all aside and just factoring in nationwide swing of votes affecting all constituencies.
How much votes you expect our oppositions to win or lose against PAP?
If our oppositions have a nationwide swing of 5 percent in their favor, would WP win Aljunied GRC?
Aljunied was the highest score for the opp with 44% but if you add that to the score you get 49% which is still a loss.
There are also conditions that change so just using a 5% formula might be inaccurate. For eg the WP team will have a lot of new faces. Who they are also plays a part. The last time they had a colorful Gomez and GMS plus a very qualified chap Tan Wui Hua who earns more than a PAP minister. If they are replaced with less profile chaps, it won't work.
In Tampines you can't just add 5% because the NSP SG GMS is now running there compared to a lacklustre team the last time. There is also Chiam's GRC and KJ's GRC (if the two do not combine).
When SDP became the only opp in 1988 it increased its prominence and won 3 in 1991. WP went the other way, lowering its prominence for whatever reasons or intentions. It might fall short of expectations.
I expect an 8% swing this time. PAP's votes will be around 58% plus minus. However WP will get less least out of the 8% because it is winning over the last batch of voters to cross 50% which is the hardest. Maybe 5% and fall short. The others will raise 10% for themselves, still falling short.