Inflation is not needed but miners need reward to keep the network secure.
If the only incentivization of mining would be transactional fees, will this currency be really secure?
Satoshi promoted the idea of merged mining
(...)
Instead of fragmentation, networks share and augment each other's total CPU power. This would solve the problem that if there are multiple networks, they are a danger to each other if the available CPU power gangs up on one. Instead, all networks in the world would share combined CPU power, increasing the total strength. It would make it easier for small networks to get started by tapping into a ready base of miners.
If in the future merged mining would gain a lot of significance, Bitcoin miners could easily make money on the sale of merged mined coins.
There would be no need for high transaction fees to maintain Bitcoin mining profitable (when block reward will reach zero).