Zero regrets bro. Unlimited 5G is also applicable on the 399 plan as well. My wife is using this Rs398 plan (bought for Rs348) with 2GB/day for 70 days in her home town, where 5G coverage is not available. There are few people in our friends & family with 4G phone, like my MIL is even now using Samsung S10 and my Kids used Pixel 3a phones till last month. So these plans are useful for them.
After upgrading to 5G phones, my Mom and Kids are now using the same Rs1559 plan. Once 5G plans are announced, they will be back to using those vouchers. There is NO expiry date for vouchers. So no need for any regrets. I am using Rs199 Postpaid plan for more than a year.