Any idea why it decides to give way? The load has remained more or less the same for many years. But  Yes they are as old as the house , more then 20 years.

Yeah but the problem could be defective wiring or connections which dont create overcurrent (fuse protect overcurrent); fuse also got slow burn type which means if u load a 60A for very short period beyond 60A it will not burn instantly/trip like a mcb.

Best is to change the whole thing to a proper set of MCB/RCCD/main isolator/RCD 10ma for water heater.

Branded ones will keep quality better. Minimum also get schneider, ABB or hager. Not that expensive la.. but best to get all in a series if possible. Cheapo MCB tarak quality one.

Basically the proper setup will be
[optional] main switch isolator
63A RCCD, 30ma recommended for safety. 100ma of 240V can cause alot of human damage. The lesser current leak thru human the safer.
63A MCB/MCCB (1phase / 3phases)
then individual MCB - typically C20 (20A) for 13A sockets, aircon, then C6 for lighting.
If u wanna further protect get a B16 or B20 (20A but B-curve) MCB exclusive for water heater circuit + 10ma RCCD. WH must have an MCB + 10ma RCCD.
Outdoor circuit (external sockets that can get wet), autogate,  ding dong bell, ideally also should be protected with 10ma RCCD.

Nope not an electrician by profession, but I did sell MCBs, motors, inverters and related automation controls (sensors, PLCs) before many years back.

DB box, matter of affordability for consumer grade units. Best is to go for metal clad boxes (can even find Eaton ones on shopee ), but any decent one will work.
Metal > PVC for flame retardant properties.

Basically the idea is that the MCB is supposed to sacrifice itself way before the wires overheat as that can cause a fire and extensive damage (imagine running new cables need to hack and replace everything). Hence I believe in properly sizing the MCB according to the load i.e. not putting an excessively large MCB for a given load (next size up is best). In my case, my place they put in a C10 (10A) MCB to protect a few lighting points which is rather overkill (even if you use all conventional fluorescent light which has initial spike when starting), it will not overload a C6 or even a 3A MCB. I also need to balance cost/sizing because other than the normal C6/10/16/20 MCBs its hard to get the other sizes affordably in MY.

WH (water heater) has ST special guidelines issued that you must protect areas with wet floor with 10ma RCCD - this will protect current imbalance only (i.e. input L and return output N must equal current else something has leaked (better get decent ones from ABB/Hager/Schneider).
Note that RCCD only protect against imbalanced current only, it does NOT protect against current overload (wire protection).

Hence WH RCCD must be run together with a normal MCB (for current overload).

Why B curve instead of C curve for WH?
As WH's heater is a resistive load (i.e. uses resistive metal to generate heat) and not inductive (ie. no motor is involved) load, B curve MCB is better as they are more sensitive (trips at lower spike current) than a C curve MCB (which is essentially a general use one as things like aircon/vacuum cleaner are motor-based, so less nuisance tripping).

RS/element14 does sell Siemens B16 MCB Siemens B16 MCB which should work for most 3.3/3.6kw heater (3300/240=13.75A, mine ran at 13.2A on clamp meter test) (3600/240 = 15A which is cutting it a bit close but still possible to use on 16A MCB). If you have very strong water pressure and using those 4.8kw models from hitachi you are running 20A nominal so a 20A would not be enough, probably a B25/C25 can be used (https://new.abb.com/products/2CDS211001R0254/miniature-circuit-breaker-sh200-1p-c-25-ampere).

https://cache.industry.siemens.com/dl/files...50852395217.pdf
see page 23. actually siemens recommends using B curve for universal use (socket outlets and lighting circuits) vs MY general practice to use C curve for everything 

https://www.se.com/au/en/faqs/FA290880/
even SE also recommends B curve for residential use.

What is B- or C- curve
https://www.se.com/in/en/faqs/FA228794/ and
https://library.e.abb.com/public/e39a8392ab...403003B0202.pdf
see page 7 trip curve info for diff between B/C/D curve

https://library.e.abb.com/public/114371fcc8...2D0201_view.pdf
see page 3, trip curve characteristics for inrush (spike) current. this means B spike is 3-5x rated current before tripping, C spike is 5-10x rated current before tripping.