actually,this is always they thought..
do you know who became a very important military outpost against USSR??It wasn't France or Britain..It was Turkey and West Germany.
against USSR,Turkey couldn't do anything significant,but they did provide a strategic foothold against USSR.
Next door neighbours always provide an excellent foothold.And It is just rhetoric to say "China is too big for any S.Asian or S.E.Asian country to confront diplomatically, militarily or economically"..As articles published few years ago,PLAN is contained within "First Island Chain" and it is possible because multiple countries backed by USA installed undersea and sub surface detection capability and continuously monitoring it.
And now Diplomatically,what they're doing is simply pushing these countries aside using force.Because they do know they can't counter it using diplomacy..but is it that hard to settle??NO.But of course each country has its "Achelis Heel",and SCS will be China's.
What you're missing is...China is actually "Locked" geographically by these countries.Their CPEC will be threatened if IN swings into action in any crisis.and these are original threats which you can't resolve with just few "Warnings".Good Relation with neighbours are generally key for prosperity.You can't achieve anything by pushing these countries,however small it is.Leverage on them is what benefits you most.