I think the following thing should consider
1.decentralied
BTC is much decentralied if all miner mine btc by p2p , but actually the top 5 mine pool can decide any thing of bts ,if they unite,
but why less people think btc is decentralied, because no upper limit of mine pool
so it is not a good idea to set a low upper limit of witnesses ,
2.efficiency and cost
as a exchange and payment system , we need high efficiency and low cost , if the the amount of actual witnesses is too larger , it would influence
efficiency , since there is one witness is working for producing block any time .
3....
so I suggest that set a larger upper limit of witnesses, like 35 or 51, but actual witnesses is not too much , eg 17 or 21
there is a mechanism to make there is a small amount of actual witnesses to keep high efficiency .
is it a trick of this mechanism? No. this is not , the high upper limit of witnessed is to keep the safety when there is attack