The fact that Sigma and Tamron haven’t supported the RF mount and the rumours heavily imply that this is Canon’s doing shows they care very much. I highly doubt Canon would be charging £2900 for the 100-500 if the below were available natively on RF with AF performance close to OEM glass.Even so why do you believe Canon would care? Haven't you seen the prices of the other RF lenses? Much more expensive than their EF counterparts. Just compare the EF 70-200 and RF 70-200 lenses or the 85 1.2L ones. Add 1.2L instead of 1.4 L for the 35mm version and you have an even more expensive lens.
Sigma 150-600 f5-6.3 Sport - £1199
Tamron 100-500 f5-6.7 - £1199
By locking out Sigma and Tamron from the RF mount sure more profits for Canon but less money in Canon shooter’s pockets.