That is why they re-examine them... not just to make sure everything is going to plan, or if it isn't then what needs to be done to make it go to plan (ie production of S-400 wasn't happening fast enough so they decided to change the plan to add an extra production plant so they could cope with the requirements), or if the plan was unrealistic then to change the plan.
Good planning requires flexibility... the longer the term planning the more flexibility it needs.
For instance in the Navy there is no sense in planning to build 4 new aircraft carriers right now. First of all there is no production capacity for that, and even if there was then those carriers would need lots of support infrastructure including upgraded ports and lots of supply ships and support ships that simply aren't ready yet.
A simple change in one area will require a check through the entire plan to look at how that effects other programs, like the upgrades of in service vessels etc etc.
The reason the LMFS was given a low priority and not given funding in the 2020 plans is because they wanted to get the PAK FA ready and into service before they even considered funding another aircraft. There were no guarantees... when the GVP was written that India would be on board, or that things would be relatively smooth... and there are problems with cracking so it hasn't been perfect. There is no rush in Russia for an F-35 like aircraft because the Su-35s and Mig-35s are available if needed and able to be produced and are very competitive and capable aircraft. Delaying the final development of the LMFS means lessons from the F-35 program can be absorbed and adopted which reduces risk.
Remember the LMFS program is not new and started out with the MFI program as the LFI program. Both became multi role programs... MFS and LFS and have evolved and changed over time, but there have been no complete and radical changes in requirements so existing designs can be modified and adapted. Both Sukhoi and Mig were working on 5th gen programs and have developed 5th gen technology... a lot of which they are putting in their -35s. Sukhoi have the advantage of actually putting theirs in service and having a 5th gen series of prototypes to fully test, but Mig will have supercomputers pounding away the numbers testing all sorts of options and combinations too.