You're absolutely spot on. However some people just want convenience at all costs, so they want automatic renewals. Also, some people want the rollover data, which you can only get if you renew on or before the expiry date of the data plan.
Everyone has their own preference.
Honestly, I just made a trip to Kuala Lumpur yesterday. The roaming partner Maxis connection really very strong, whereas the heya parent Singtel connection also very strong. Even though I have been posting IG stories on the moving bus, the upload quality still very crystal clear nonetheless.
The only setback is that yesterday, because spent a whopping 13 hours on the bus (7 hours to KL and 6 hours to SG), I inevitably used close to 600MB of roaming data. But anyway, every subsequent topup for roaming connection only $3. Considered 一分錢一分貨吧. Really what we pay is what we get. $3 is small money.
In conclusion, even if don't have automated renewal, manually buy new plan also quite easy.
Just like what you have said, Singtel is the strongest telco in Singapore, because it has the longest history in Singapore.
Able to get Singtel is really a privilege. Really thankful that Singtel has the sub brands Gomo and Heya to reach out to the lower income customers who could not afford the more pricey Singtel plans.