I think its fine how it is. I started as an FI(R) instructing at weekends about 6 months ago. I am approaching the 100hrs FI but do not have anywhere near the requisite number of sign offs yet.
I feel far from ready to be in a position to decide whether someone is in a position to go solo for the first time so I think the regulators have got it about right. It is a big responsibility sending someone of solo - even if it is for their fourth of fifth, let alone the first.
Most of the students I have taken over were at the start of their PPL so they are just coming up to first solo stage now, so although I have been instructing six months and nearly have the hours I don't have all the experience I need I feel to go unrestricted.
To my mind the 100hrs/25 sign offs is right as it balances instructional time with sending people solo. If when I had started I had 4 students at the circuit stage I could have had the sign offs done in three months but be lacking experience in other areas.
Still got much to learn.