reactivation = scan the OR again and add mobile plan.
you need to do it everytime u change device too. see the steps for a iPhone user to transfer esim to a new iphone. it is certainly not as straightforward as swapping a physical sim to another phone and use it straightaway, hassle free.
one can ask himself, how often u will change ur main line mobile plan? most people will likely to stick to it for years like at least two years if u have mobile contract with telco. pretty much within this two years u will never even change it to another phone. this is why esim for main line is good.
how often u will probably need to change second sim if u visit different countries within the two years? probably a few time a year?
one is one time for very long time, one is few times a year probably depending on how often u go overseas. most importantly, if u need to JB tmr for example, can u confirm that you can get hold of a esim the local telco there to use immediately? impossible. likely u cannot confirm if esim is even available there. for your info, in lion city, some telcos have esim but only for POSTPAID and not prepaid, if u are foreigner, u are unlikely to get a esim in a foreign too. i believe this should be the same for a lot of other countries.
would u rather deal with uncertainties like this or u would rather reserve ur physical sim card slot for overseas prepaid physical sim card use? the answer should be obvious.
i dun see a logic why some are saying the other ways round.
u go to all red and green telcos website, u can see many many are asking for esim, and this is for this reason. if the logic of using esim as secondary works for them, people will not even ask for it.
and yes, if u are iphone users, u need to reactivate ur facetime and imessage too, and it costs $$ to reactivate for certain telcos too. since it is main line, why one needs to go thru the hassle of this. why the prepaid sim card u get from overseas, u will never need to enable facetime or imessage for the numbers.
Just to make sure we are on the same page:
You can store multiple eSIM profiles in the same device (depends on manufacturer, but lowest I've heard is 8 for phones at any one time, which is plenty), and switching between them do not need to rescan QR code. You only need to rescan QR code if and only if you deleted it from the device.
iMessage/Facetime activation charge happens whenever new line is introduced, be it changing between eSIM profiles, or swapping SIM cards, if you accept the registration. Both lines will have to reregister when that happens, unless you specify which line to register only when setting up a brand new SIM (eSIM profile / physical SIM).
I've M1, Circles, Ubigi and Yoodo eSIM on my iPhone 11 Pro Max and switches between some of them regularly, while having StarHub Physical SIM inside the device too, sometimes take out to put in other telco SIMs (eg TPG).
I won't say whether main line in eSIM or physical SIM format is correct or wrong choice, but my preference is for it to still be physical SIM. If the next generation (or two) iPhone do require eSIM only, network operators worldwide do need to accept on their network asap, and putting in tourist eSIM won't be as confusing as now already.
May even push those global eSIM companies (eg Airalo) out of business, as their funds ran low due to correct pandemic and basically dry as lesser people get their plans for their next trip and opt for direct contact with the local providers instead.
I don't mind the physical SIM hassle, as I can always bring an older phone as backup anyway. Yes, not everyone has the luxury to do that, but the counterargument is that since you bought an iPhone or Pixel or flagship Galaxy devices, why don't you even have any money to purchase an old ex-flagship phone? No need to be few hundred dollars, you can get something like Galaxy S8 or iPhone 7 for not that much nowadays.
