If a new client joins and they start syncing from the latest pruning block, how does the node know that the balances in a pruning block are correct? A miner could mine an invalid pruning block with incorrect balances but then how would a new client know that that pruning block is invalid? How can it determine this without trusting anyone like how currently full nodes do not trust anyone.
If a miner submits a pruning block with modified balances, they will be rejected by the rest of the mining community as an invalid block because the submitted balances do not match what they have, and so it will never make it into the actual blockchain to stay. Thus, similar to accepting payments you should not accept a parsing block until it is 6 or more transactions old (and more is probably preferable) if you are downloading from scratch.
Also, bitcoin doesn't work on balances but rather transaction outputs. The way this would work with less complex changes would be to create new UTXOs with the balances. However, this essentially gives miners the ability to spend other people's bitcoin.
Yes, the pruning block would essentially include a copy of all UTXOs as of the previous block. However, any pruning block submitted with extra, incorrect, or missing UTXO's should automatically be rejected as an invalid pruning block, so it would not give miners the ability to spend other people's bitcoin (again those would be rejected by other miners).
Additionally disk space is becoming a lot cheaper and network bandwidth is also expected to increase. We won't be having to manage a full blockchain with current technology, technology will advance and the blockchain should not outpace technology growth as long as there i is a block size limit.
I surely hope technology keeps up. However, if the
Blockchain Size continues doubling every year as it has been, it will quickly catch up to Moore's law as that only doubles every 2 years. Hopefully the economy reaches user-saturation before then at which point blockchain growth will be linear, but if not as you say the block-size limit prevents this from becoming an issue. The main problem there is that exponentially more people each year will be unable to perform transactions, which is bad for the currency.