There is nothing wrong with following the current Rotax maintance manual for life limitations and maintenance periods.
Replacing earlier than their limit is opening yourself up to maintenance-induced failure, as you mention. But, people do this for other reasons like expediency; "while they are at it", or costs. It is probably cheaper to replace the spark plug than inspect and clean it if you are paying someone to inspect and clean.
Replacing later than their limit would be running an experiment for how much later. Which brings up condition-based maintenance:
To be safe about condition-based maintenance you need a specification and limitations to determine condition. Rotax does not provide this, so making up your own limits is, again, experimenting. If you like to experiment, you are welcome in the E-LSA category.
The previous engine failure, you mention, obviously had something going wrong that could not be seen and had nothing to do with spark plug or oil changes. Most maintenance or condition inspections do not get inside the engine to inspect for damage, wear, or fatigue.
From a purely anecdotal perspective on the oil change, 100 hours at 112 knots typical cruise is almost 13k miles. Many cars are set up for 10k to 12k miles per oil change, which takes between 150 and 300 hours to accomplish, typically. I don't see that being outside convention. For years, everyone said you had to change your oil at 3k miles. This was good advertising by the oil change places and the oil companies to get you to spend money. You will still find people doing it, but there is no technical basis for doing so.
Mike Busch mentions most of the aircraft engines fail to make TBO due to corrosion (just sitting). You fly so much, I would not worry about it or going to the Rotax limits.