I doubt RSAF will keep the F15Sgs or the post mid-life updated F15Sgs for so long as it will be outpaced by technology.
What is happening is that each of the royal Singapore armed forces 4 services i.e. army, RSAF, RSN and Digital/Cyber warfare and Intelligence has a list of projects in an order of priority.
RSN for example is focused on the Multi Role Combat Vessels which will likely be the most sophisticated and powerful surface combatant in South East Asia when introduced in 2025 to 2030, mid life update of the Formidable frigates and new light frigates/OPVs amidst integrating the new Invincible class submarines.
RSAF is likely focused on completing the upgrades of the F16C/Ds to F16Vs + phased purchase of F35A/Bs in batches + completion of deliveries of the Chinooks and Cougar helicopters + new MPAs (with the navy) before it focuses on other planned developments.
An example of a development which took years of focus and is largely completed is the Island Defence System which saw the I hawk missiles replaced by Aster 30 Blk 1 missile systems, Rapier missiles replaced by Spyder SR missile systems, new Bolide MK2 missiles, new CRAM systems (likely Iron Dome if you care to connect the sub systems and Sgs requirements) and integration of all these systems into a one-stop performing system.
Even with a 2023 USD $13.4 billion budget, Singapore could not do everything at once. SAFs projects take years to complete.
Rsaf f-15sg will likely remain effective against adversaries in the next few decades especially in strike role, will likely be upgraded.