Anyone who reads up on this topic will realize that this is another trend in response to, for example, rising construction costs.
With all due respect, if there really is a completely absurd "trend" to circumvent the building obligation with tiny homes, this will be taken into account in the corresponding regulations in the future.
The building obligation is not only meant to prevent speculation but also to contribute to creating housing that is often lacking in many places. A 15sqm wooden shack definitely does not fulfill this.
What you write is conceptually questionable anyway. Tiny houses are not free, and if building becomes more and more expensive, it is pointless to...
1. waste a lot of money on an outbuilding
2. wait even longer
3. A tiny house replaces living space for one person, not for a multi-person family; they would then have to build several tiny houses, which would again be more expensive than a traditional house since each time you need separate heating & electrical installations, or how do you imagine that?